Merge pull request #9050 from LabNConsulting/chopps/reset-parallel

[mirror_frr.git] / doc / user / bgp.rst

.. _bgp:

***
BGP
***

:abbr:`BGP` stands for Border Gateway Protocol. The latest BGP version is 4.
BGP-4 is one of the Exterior Gateway Protocols and the de facto standard
interdomain routing protocol. BGP-4 is described in :rfc:`1771` and updated by
:rfc:`4271`. :rfc:`2858` adds multiprotocol support to BGP-4.

.. _starting-bgp:

Starting BGP
============

The default configuration file of *bgpd* is :file:`bgpd.conf`. *bgpd* searches
the current directory first, followed by |INSTALL_PREFIX_ETC|/bgpd.conf. All of
*bgpd*'s commands must be configured in :file:`bgpd.conf` when the integrated
config is not being used.

*bgpd* specific invocation options are described below. Common options may also
be specified (:ref:`common-invocation-options`).

.. program:: bgpd

.. option:: -p, --bgp_port <port>

   Set the bgp protocol's port number. When port number is 0, that means do not
   listen bgp port.

.. option:: -l, --listenon

   Specify specific IP addresses for bgpd to listen on, rather than its default
   of ``0.0.0.0`` / ``::``. This can be useful to constrain bgpd to an internal
   address, or to run multiple bgpd processes on one host. Multiple addresses
   can be specified.

   In the following example, bgpd is started listening for connections on the
   addresses 100.0.1.2 and fd00::2:2. The options -d (runs in daemon mode) and
   -f (uses specific configuration file) are also used in this example as we
   are likely to run multiple bgpd instances, each one with different
   configurations, when using -l option.

   Note that this option implies the --no_kernel option, and no learned routes will be installed into the linux kernel.

.. code-block:: shell

   # /usr/lib/frr/bgpd -d -f /some-folder/bgpd.conf -l 100.0.1.2 -l fd00::2:2

.. option:: -n, --no_kernel

   Do not install learned routes into the linux kernel.  This option is useful
   for a route-reflector environment or if you are running multiple bgp
   processes in the same namespace.  This option is different than the --no_zebra
   option in that a ZAPI connection is made.

   This option can also be toggled during runtime by using the
   ``[no] bgp no-rib`` commands in VTY shell.

   Note that this option will persist after saving the configuration during
   runtime, unless unset by the ``no bgp no-rib`` command in VTY shell prior to
   a configuration write operation.

.. option:: -S, --skip_runas

   Skip the normal process of checking capabilities and changing user and group
   information.

.. option:: -e, --ecmp

   Run BGP with a limited ecmp capability, that is different than what BGP
   was compiled with.  The value specified must be greater than 0 and less
   than or equal to the MULTIPATH_NUM specified on compilation.

.. option:: -Z, --no_zebra

   Do not communicate with zebra at all.  This is different than the --no_kernel
   option in that we do not even open a ZAPI connection to the zebra process.

.. option:: -s, --socket_size

   When opening tcp connections to our peers, set the socket send buffer
   size that the kernel will use for the peers socket.  This option
   is only really useful at a very large scale.  Experimentation should
   be done to see if this is helping or not at the scale you are running
   at.

LABEL MANAGER
-------------

.. option:: -I, --int_num

   Set zclient id. This is required when using Zebra label manager in proxy mode.

.. _bgp-basic-concepts:

Basic Concepts
==============

.. _bgp-autonomous-systems:

Autonomous Systems
------------------

From :rfc:`1930`:

   An AS is a connected group of one or more IP prefixes run by one or more
   network operators which has a SINGLE and CLEARLY DEFINED routing policy.

Each AS has an identifying number associated with it called an :abbr:`ASN
(Autonomous System Number)`. This is a two octet value ranging in value from 1
to 65535. The AS numbers 64512 through 65535 are defined as private AS numbers.
Private AS numbers must not be advertised on the global Internet.

The :abbr:`ASN (Autonomous System Number)` is one of the essential elements of
BGP. BGP is a distance vector routing protocol, and the AS-Path framework
provides distance vector metric and loop detection to BGP.

.. seealso:: :rfc:`1930`

.. _bgp-address-families:

Address Families
----------------

Multiprotocol extensions enable BGP to carry routing information for multiple
network layer protocols. BGP supports an Address Family Identifier (AFI) for
IPv4 and IPv6. Support is also provided for multiple sets of per-AFI
information via the BGP Subsequent Address Family Identifier (SAFI). FRR
supports SAFIs for unicast information, labeled information (:rfc:`3107` and
:rfc:`8277`), and Layer 3 VPN information (:rfc:`4364` and :rfc:`4659`).

.. _bgp-route-selection:

Route Selection
---------------

The route selection process used by FRR's BGP implementation uses the following
decision criterion, starting at the top of the list and going towards the
bottom until one of the factors can be used.

1. **Weight check**

   Prefer higher local weight routes to lower routes.

2. **Local preference check**

   Prefer higher local preference routes to lower.

3. **Local route check**

   Prefer local routes (statics, aggregates, redistributed) to received routes.

4. **AS path length check**

   Prefer shortest hop-count AS_PATHs.

5. **Origin check**

   Prefer the lowest origin type route. That is, prefer IGP origin routes to
   EGP, to Incomplete routes.

6. **MED check**

   Where routes with a MED were received from the same AS, prefer the route
   with the lowest MED. :ref:`bgp-med`.

7. **External check**

   Prefer the route received from an external, eBGP peer over routes received
   from other types of peers.

8. **IGP cost check**

   Prefer the route with the lower IGP cost.

9. **Multi-path check**

   If multi-pathing is enabled, then check whether the routes not yet
   distinguished in preference may be considered equal. If
   :clicmd:`bgp bestpath as-path multipath-relax` is set, all such routes are
   considered equal, otherwise routes received via iBGP with identical AS_PATHs
   or routes received from eBGP neighbours in the same AS are considered equal.

10. **Already-selected external check**

    Where both routes were received from eBGP peers, then prefer the route
    which is already selected. Note that this check is not applied if
    :clicmd:`bgp bestpath compare-routerid` is configured. This check can
    prevent some cases of oscillation.

11. **Router-ID check**

    Prefer the route with the lowest `router-ID`. If the route has an
    `ORIGINATOR_ID` attribute, through iBGP reflection, then that router ID is
    used, otherwise the `router-ID` of the peer the route was received from is
    used.

12. **Cluster-List length check**

    The route with the shortest cluster-list length is used. The cluster-list
    reflects the iBGP reflection path the route has taken.

13. **Peer address**

    Prefer the route received from the peer with the higher transport layer
    address, as a last-resort tie-breaker.

.. _bgp-capability-negotiation:

Capability Negotiation
----------------------

When adding IPv6 routing information exchange feature to BGP. There were some
proposals. :abbr:`IETF (Internet Engineering Task Force)`
:abbr:`IDR (Inter Domain Routing)` adopted a proposal called Multiprotocol
Extension for BGP. The specification is described in :rfc:`2283`. The protocol
does not define new protocols. It defines new attributes to existing BGP. When
it is used exchanging IPv6 routing information it is called BGP-4+. When it is
used for exchanging multicast routing information it is called MBGP.

*bgpd* supports Multiprotocol Extension for BGP. So if a remote peer supports
the protocol, *bgpd* can exchange IPv6 and/or multicast routing information.

Traditional BGP did not have the feature to detect a remote peer's
capabilities, e.g. whether it can handle prefix types other than IPv4 unicast
routes. This was a big problem using Multiprotocol Extension for BGP in an
operational network. :rfc:`2842` adopted a feature called Capability
Negotiation. *bgpd* use this Capability Negotiation to detect the remote peer's
capabilities. If a peer is only configured as an IPv4 unicast neighbor, *bgpd*
does not send these Capability Negotiation packets (at least not unless other
optional BGP features require capability negotiation).

By default, FRR will bring up peering with minimal common capability for the
both sides. For example, if the local router has unicast and multicast
capabilities and the remote router only has unicast capability the local router
will establish the connection with unicast only capability. When there are no
common capabilities, FRR sends Unsupported Capability error and then resets the
connection.

.. _bgp-router-configuration:

BGP Router Configuration
========================

ASN and Router ID
-----------------

First of all you must configure BGP router with the :clicmd:`router bgp ASN`
command. The AS number is an identifier for the autonomous system. The BGP
protocol uses the AS number for detecting whether the BGP connection is
internal or external.

.. clicmd:: router bgp ASN

   Enable a BGP protocol process with the specified ASN. After
   this statement you can input any `BGP Commands`.

.. clicmd:: bgp router-id A.B.C.D

   This command specifies the router-ID. If *bgpd* connects to *zebra* it gets
   interface and address information. In that case default router ID value is
   selected as the largest IP Address of the interfaces. When `router zebra` is
   not enabled *bgpd* can't get interface information so `router-id` is set to
   0.0.0.0. So please set router-id by hand.


.. _bgp-multiple-autonomous-systems:

Multiple Autonomous Systems
---------------------------

FRR's BGP implementation is capable of running multiple autonomous systems at
once. Each configured AS corresponds to a :ref:`zebra-vrf`. In the past, to get
the same functionality the network administrator had to run a new *bgpd*
process; using VRFs allows multiple autonomous systems to be handled in a
single process.

When using multiple autonomous systems, all router config blocks after the
first one must specify a VRF to be the target of BGP's route selection. This
VRF must be unique within respect to all other VRFs being used for the same
purpose, i.e. two different autonomous systems cannot use the same VRF.
However, the same AS can be used with different VRFs.

.. note::

   The separated nature of VRFs makes it possible to peer a single *bgpd*
   process to itself, on one machine. Note that this can be done fully within
   BGP without a corresponding VRF in the kernel or Zebra, which enables some
   practical use cases such as :ref:`route reflectors <bgp-route-reflector>`
   and route servers.

Configuration of additional autonomous systems, or of a router that targets a
specific VRF, is accomplished with the following command:

.. clicmd:: router bgp ASN vrf VRFNAME

   ``VRFNAME`` is matched against VRFs configured in the kernel. When ``vrf
   VRFNAME`` is not specified, the BGP protocol process belongs to the default
   VRF.

An example configuration with multiple autonomous systems might look like this:

.. code-block:: frr

   router bgp 1
    neighbor 10.0.0.1 remote-as 20
    neighbor 10.0.0.2 remote-as 30
   !
   router bgp 2 vrf blue
    neighbor 10.0.0.3 remote-as 40
    neighbor 10.0.0.4 remote-as 50
   !
   router bgp 3 vrf red
    neighbor 10.0.0.5 remote-as 60
    neighbor 10.0.0.6 remote-as 70
   ...

.. seealso:: :ref:`bgp-vrf-route-leaking`
.. seealso:: :ref:`zebra-vrf`


.. _bgp-views:

Views
-----

In addition to supporting multiple autonomous systems, FRR's BGP implementation
also supports *views*.

BGP views are almost the same as normal BGP processes, except that routes
selected by BGP are not installed into the kernel routing table.  Each BGP view
provides an independent set of routing information which is only distributed
via BGP. Multiple views can be supported, and BGP view information is always
independent from other routing protocols and Zebra/kernel routes. BGP views use
the core instance (i.e., default VRF) for communication with peers.

.. clicmd:: router bgp AS-NUMBER view NAME

   Make a new BGP view. You can use an arbitrary word for the ``NAME``. Routes
   selected by the view are not installed into the kernel routing table.

   With this command, you can setup Route Server like below.

   .. code-block:: frr

      !
      router bgp 1 view 1
       neighbor 10.0.0.1 remote-as 2
       neighbor 10.0.0.2 remote-as 3
      !
      router bgp 2 view 2
       neighbor 10.0.0.3 remote-as 4
       neighbor 10.0.0.4 remote-as 5

.. clicmd:: show [ip] bgp view NAME

   Display the routing table of BGP view ``NAME``.


Route Selection
---------------

.. clicmd:: bgp bestpath as-path confed

   This command specifies that the length of confederation path sets and
   sequences should should be taken into account during the BGP best path
   decision process.

.. clicmd:: bgp bestpath as-path multipath-relax

   This command specifies that BGP decision process should consider paths
   of equal AS_PATH length candidates for multipath computation. Without
   the knob, the entire AS_PATH must match for multipath computation.

.. clicmd:: bgp bestpath compare-routerid

   Ensure that when comparing routes where both are equal on most metrics,
   including local-pref, AS_PATH length, IGP cost, MED, that the tie is broken
   based on router-ID.

   If this option is enabled, then the already-selected check, where
   already selected eBGP routes are preferred, is skipped.

   If a route has an `ORIGINATOR_ID` attribute because it has been reflected,
   that `ORIGINATOR_ID` will be used. Otherwise, the router-ID of the peer the
   route was received from will be used.

   The advantage of this is that the route-selection (at this point) will be
   more deterministic. The disadvantage is that a few or even one lowest-ID
   router may attract all traffic to otherwise-equal paths because of this
   check. It may increase the possibility of MED or IGP oscillation, unless
   other measures were taken to avoid these. The exact behaviour will be
   sensitive to the iBGP and reflection topology.

.. clicmd:: bgp bestpath peer-type multipath-relax

   This command specifies that BGP decision process should consider paths
   from all peers for multipath computation. If this option is enabled,
   paths learned from any of eBGP, iBGP, or confederation neighbors will
   be multipath if they are otherwise considered equal cost.

.. clicmd:: maximum-paths (1-128)

   Sets the maximum-paths value used for ecmp calculations for this
   bgp instance in EBGP.  The maximum value listed, 128, can be limited by
   the ecmp cli for bgp or if the daemon was compiled with a lower
   ecmp value.  This value can also be set in ipv4/ipv6 unicast/labeled
   unicast to only affect those particular afi/safi's.

.. clicmd:: maximum-paths ibgp (1-128) [equal-cluster-length]

   Sets the maximum-paths value used for ecmp calculations for this
   bgp instance in IBGP.  The maximum value listed, 128, can be limited by
   the ecmp cli for bgp or if the daemon was compiled with a lower
   ecmp value.  This value can also be set in ipv4/ipv6 unicast/labeled
   unicast to only affect those particular afi/safi's.

.. _bgp-distance:

Administrative Distance Metrics
-------------------------------

.. clicmd:: distance bgp (1-255) (1-255) (1-255)

   This command change distance value of BGP. The arguments are the distance
   values for for external routes, internal routes and local routes
   respectively.

.. clicmd:: distance (1-255) A.B.C.D/M

.. clicmd:: distance (1-255) A.B.C.D/M WORD

   Sets the administrative distance for a particular route.

.. _bgp-requires-policy:

Require policy on EBGP
-------------------------------

.. clicmd:: bgp ebgp-requires-policy

   This command requires incoming and outgoing filters to be applied
   for eBGP sessions as part of RFC-8212 compliance. Without the incoming
   filter, no routes will be accepted. Without the outgoing filter, no
   routes will be announced.

   This is enabled by default for the traditional configuration and
   turned off by default for datacenter configuration.

   When you enable/disable this option you MUST clear the session.

   When the incoming or outgoing filter is missing you will see
   "(Policy)" sign under ``show bgp summary``:

   .. code-block:: frr

      exit1# show bgp summary

      IPv4 Unicast Summary (VRF default):
      BGP router identifier 10.10.10.1, local AS number 65001 vrf-id 0
      BGP table version 4
      RIB entries 7, using 1344 bytes of memory
      Peers 2, using 43 KiB of memory

      Neighbor        V         AS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt
      192.168.0.2     4      65002         8        10        0    0    0 00:03:09            5 (Policy)
      fe80:1::2222    4      65002         9        11        0    0    0 00:03:09     (Policy) (Policy)

   Additionally a `show bgp neighbor` command would indicate in the `For address family:`
   block that:

   .. code-block:: frr

      exit1# show bgp neighbor
      ...
      For address family: IPv4 Unicast
       Update group 1, subgroup 1
       Packet Queue length 0
       Inbound soft reconfiguration allowed
       Community attribute sent to this neighbor(all)
       Inbound updates discarded due to missing policy
       Outbound updates discarded due to missing policy
       0 accepted prefixes

Reject routes with AS_SET or AS_CONFED_SET types
------------------------------------------------

.. clicmd:: bgp reject-as-sets

   This command enables rejection of incoming and outgoing routes having AS_SET or AS_CONFED_SET type.

Suppress duplicate updates
--------------------------

.. clicmd:: bgp suppress-duplicates

   For example, BGP routers can generate multiple identical announcements with
   empty community attributes if stripped at egress. This is an undesired behavior.
   Suppress duplicate updates if the route actually not changed.
   Default: enabled.

Disable checking if nexthop is connected on EBGP sessions
---------------------------------------------------------

.. clicmd:: bgp disable-ebgp-connected-route-check

   This command is used to disable the connection verification process for EBGP peering sessions
   that are reachable by a single hop but are configured on a loopback interface or otherwise
   configured with a non-directly connected IP address.

.. _bgp-route-flap-dampening:

Route Flap Dampening
--------------------

.. clicmd:: bgp dampening [(1-45) [(1-20000) (1-20000) (1-255)]]

   This command enables (with optionally specified dampening parameters) or
   disables route-flap dampening for all routes of a BGP instance.

.. clicmd:: neighbor PEER dampening [(1-45) [(1-20000) (1-20000) (1-255)]]

   This command enables (with optionally specified dampening parameters) or
   disables route-flap dampening for all routes learned from a BGP peer.

.. clicmd:: neighbor GROUP dampening [(1-45) [(1-20000) (1-20000) (1-255)]]

   This command enables (with optionally specified dampening parameters) or
   disables route-flap dampening for all routes learned from peers of a peer
   group.

   half-life
      Half-life time for the penalty in minutes (default value: 15).

   reuse-threshold
      Value to start reusing a route (default value: 750).

   suppress-threshold
      Value to start suppressing a route (default value: 2000).

   max-suppress
      Maximum duration to suppress a stable route in minutes (default value:
      60).

   The route-flap damping algorithm is compatible with :rfc:`2439`. The use of
   these commands is not recommended nowadays.

   At the moment, route-flap dampening is not working per VRF and is working only
   for IPv4 unicast and multicast.

   With different parameter sets configurable for BGP instances, peer groups and
   peers, the active dampening profile for a route is chosen on the fly,
   allowing for various changes in configuration (i.e. peer group memberships)
   during runtime. The parameter sets are taking precedence in the following
   order:

   1. Peer
   2. Peer group
   3. BGP instance

   The negating commands do not allow to exclude a peer/peer group from a peer
   group/BGP instances configuration.

.. seealso::
   https://www.ripe.net/publications/docs/ripe-378

.. _bgp-med:

Multi-Exit Discriminator
------------------------

The BGP :abbr:`MED (Multi-Exit Discriminator)` attribute has properties which
can cause subtle convergence problems in BGP. These properties and problems
have proven to be hard to understand, at least historically, and may still not
be widely understood. The following attempts to collect together and present
what is known about MED, to help operators and FRR users in designing and
configuring their networks.

The BGP :abbr:`MED` attribute is intended to allow one AS to indicate its
preferences for its ingress points to another AS. The MED attribute will not be
propagated on to another AS by the receiving AS - it is 'non-transitive' in the
BGP sense.

E.g., if AS X and AS Y have 2 different BGP peering points, then AS X might set
a MED of 100 on routes advertised at one and a MED of 200 at the other. When AS
Y selects between otherwise equal routes to or via AS X, AS Y should prefer to
take the path via the lower MED peering of 100 with AS X. Setting the MED
allows an AS to influence the routing taken to it within another, neighbouring
AS.

In this use of MED it is not really meaningful to compare the MED value on
routes where the next AS on the paths differs. E.g., if AS Y also had a route
for some destination via AS Z in addition to the routes from AS X, and AS Z had
also set a MED, it wouldn't make sense for AS Y to compare AS Z's MED values to
those of AS X. The MED values have been set by different administrators, with
different frames of reference.

The default behaviour of BGP therefore is to not compare MED values across
routes received from different neighbouring ASes. In FRR this is done by
comparing the neighbouring, left-most AS in the received AS_PATHs of the routes
and only comparing MED if those are the same.

Unfortunately, this behaviour of MED, of sometimes being compared across routes
and sometimes not, depending on the properties of those other routes, means MED
can cause the order of preference over all the routes to be undefined. That is,
given routes A, B, and C, if A is preferred to B, and B is preferred to C, then
a well-defined order should mean the preference is transitive (in the sense of
orders [#med-transitivity-rant]_) and that A would be preferred to C.

However, when MED is involved this need not be the case. With MED it is
possible that C is actually preferred over A. So A is preferred to B, B is
preferred to C, but C is preferred to A. This can be true even where BGP
defines a deterministic 'most preferred' route out of the full set of A,B,C.
With MED, for any given set of routes there may be a deterministically
preferred route, but there need not be any way to arrange them into any order
of preference. With unmodified MED, the order of preference of routes literally
becomes undefined.

That MED can induce non-transitive preferences over routes can cause issues.
Firstly, it may be perceived to cause routing table churn locally at speakers;
secondly, and more seriously, it may cause routing instability in iBGP
topologies, where sets of speakers continually oscillate between different
paths.

The first issue arises from how speakers often implement routing decisions.
Though BGP defines a selection process that will deterministically select the
same route as best at any given speaker, even with MED, that process requires
evaluating all routes together. For performance and ease of implementation
reasons, many implementations evaluate route preferences in a pair-wise fashion
instead. Given there is no well-defined order when MED is involved, the best
route that will be chosen becomes subject to implementation details, such as
the order the routes are stored in. That may be (locally) non-deterministic,
e.g.: it may be the order the routes were received in.

This indeterminism may be considered undesirable, though it need not cause
problems. It may mean additional routing churn is perceived, as sometimes more
updates may be produced than at other times in reaction to some event .

This first issue can be fixed with a more deterministic route selection that
ensures routes are ordered by the neighbouring AS during selection.
:clicmd:`bgp deterministic-med`. This may reduce the number of updates as routes
are received, and may in some cases reduce routing churn. Though, it could
equally deterministically produce the largest possible set of updates in
response to the most common sequence of received updates.

A deterministic order of evaluation tends to imply an additional overhead of
sorting over any set of n routes to a destination. The implementation of
deterministic MED in FRR scales significantly worse than most sorting
algorithms at present, with the number of paths to a given destination.  That
number is often low enough to not cause any issues, but where there are many
paths, the deterministic comparison may quickly become increasingly expensive
in terms of CPU.

Deterministic local evaluation can *not* fix the second, more major, issue of
MED however. Which is that the non-transitive preference of routes MED can
cause may lead to routing instability or oscillation across multiple speakers
in iBGP topologies. This can occur with full-mesh iBGP, but is particularly
problematic in non-full-mesh iBGP topologies that further reduce the routing
information known to each speaker. This has primarily been documented with iBGP
:ref:`route-reflection <bgp-route-reflector>` topologies. However, any
route-hiding technologies potentially could also exacerbate oscillation with MED.

This second issue occurs where speakers each have only a subset of routes, and
there are cycles in the preferences between different combinations of routes -
as the undefined order of preference of MED allows - and the routes are
distributed in a way that causes the BGP speakers to 'chase' those cycles. This
can occur even if all speakers use a deterministic order of evaluation in route
selection.

E.g., speaker 4 in AS A might receive a route from speaker 2 in AS X, and from
speaker 3 in AS Y; while speaker 5 in AS A might receive that route from
speaker 1 in AS Y. AS Y might set a MED of 200 at speaker 1, and 100 at speaker
3. I.e, using ASN:ID:MED to label the speakers:

::

   .
             /---------------\\
   X:2------|--A:4-------A:5--|-Y:1:200
               Y:3:100--|-/   |
             \\---------------/



Assuming all other metrics are equal (AS_PATH, ORIGIN, 0 IGP costs), then based
on the RFC4271 decision process speaker 4 will choose X:2 over Y:3:100, based
on the lower ID of 2. Speaker 4 advertises X:2 to speaker 5.  Speaker 5 will
continue to prefer Y:1:200 based on the ID, and advertise this to speaker 4.
Speaker 4 will now have the full set of routes, and the Y:1:200 it receives
from 5 will beat X:2, but when speaker 4 compares Y:1:200 to Y:3:100 the MED
check now becomes active as the ASes match, and now Y:3:100 is preferred.
Speaker 4 therefore now advertises Y:3:100 to 5, which will also agrees that
Y:3:100 is preferred to Y:1:200, and so withdraws the latter route from 4.
Speaker 4 now has only X:2 and Y:3:100, and X:2 beats Y:3:100, and so speaker 4
implicitly updates its route to speaker 5 to X:2. Speaker 5 sees that Y:1:200
beats X:2 based on the ID, and advertises Y:1:200 to speaker 4, and the cycle
continues.

The root cause is the lack of a clear order of preference caused by how MED
sometimes is and sometimes is not compared, leading to this cycle in the
preferences between the routes:

::

   .
    /---> X:2 ---beats---> Y:3:100 --\\
   |                                   |
   |                                   |
    \\---beats--- Y:1:200 <---beats---/



This particular type of oscillation in full-mesh iBGP topologies can  be
avoided by speakers preferring already selected, external routes rather than
choosing to update to new a route based on a post-MED metric (e.g.  router-ID),
at the cost of a non-deterministic selection process. FRR implements this, as
do many other implementations, so long as it is not overridden by setting
:clicmd:`bgp bestpath compare-routerid`, and see also
:ref:`bgp-route-selection`.

However, more complex and insidious cycles of oscillation are possible with
iBGP route-reflection, which are not so easily avoided. These have been
documented in various places. See, e.g.:

- [bgp-route-osci-cond]_
- [stable-flexible-ibgp]_
- [ibgp-correctness]_

for concrete examples and further references.

There is as of this writing *no* known way to use MED for its original purpose;
*and* reduce routing information in iBGP topologies; *and* be sure to avoid the
instability problems of MED due the non-transitive routing preferences it can
induce; in general on arbitrary networks.

There may be iBGP topology specific ways to reduce the instability risks, even
while using MED, e.g.: by constraining the reflection topology and by tuning
IGP costs between route-reflector clusters, see :rfc:`3345` for details.  In the
near future, the Add-Path extension to BGP may also solve MED oscillation while
still allowing MED to be used as intended, by distributing "best-paths per
neighbour AS". This would be at the cost of distributing at least as many
routes to all speakers as a full-mesh iBGP would, if not more, while also
imposing similar CPU overheads as the "Deterministic MED" feature at each
Add-Path reflector.

More generally, the instability problems that MED can introduce on more
complex, non-full-mesh, iBGP topologies may be avoided either by:

- Setting :clicmd:`bgp always-compare-med`, however this allows MED to be compared
  across values set by different neighbour ASes, which may not produce
  coherent desirable results, of itself.
- Effectively ignoring MED by setting MED to the same value (e.g.: 0) using
  :clicmd:`set metric METRIC` on all received routes, in combination with
  setting :clicmd:`bgp always-compare-med` on all speakers. This is the simplest
  and most performant way to avoid MED oscillation issues, where an AS is happy
  not to allow neighbours to inject this problematic metric.

As MED is evaluated after the AS_PATH length check, another possible use for
MED is for intra-AS steering of routes with equal AS_PATH length, as an
extension of the last case above. As MED is evaluated before IGP metric, this
can allow cold-potato routing to be implemented to send traffic to preferred
hand-offs with neighbours, rather than the closest hand-off according to the
IGP metric.

Note that even if action is taken to address the MED non-transitivity issues,
other oscillations may still be possible. E.g., on IGP cost if iBGP and IGP
topologies are at cross-purposes with each other - see the Flavel and Roughan
paper above for an example. Hence the guideline that the iBGP topology should
follow the IGP topology.

.. clicmd:: bgp deterministic-med

   Carry out route-selection in way that produces deterministic answers
   locally, even in the face of MED and the lack of a well-defined order of
   preference it can induce on routes. Without this option the preferred route
   with MED may be determined largely by the order that routes were received
   in.

   Setting this option will have a performance cost that may be noticeable when
   there are many routes for each destination. Currently in FRR it is
   implemented in a way that scales poorly as the number of routes per
   destination increases.

   The default is that this option is not set.

Note that there are other sources of indeterminism in the route selection
process, specifically, the preference for older and already selected routes
from eBGP peers, :ref:`bgp-route-selection`.

.. clicmd:: bgp always-compare-med

   Always compare the MED on routes, even when they were received from
   different neighbouring ASes. Setting this option makes the order of
   preference of routes more defined, and should eliminate MED induced
   oscillations.

   If using this option, it may also be desirable to use
   :clicmd:`set metric METRIC` to set MED to 0 on routes received from external
   neighbours.

   This option can be used, together with :clicmd:`set metric METRIC` to use
   MED as an intra-AS metric to steer equal-length AS_PATH routes to, e.g.,
   desired exit points.


.. _bgp-graceful-restart:

Graceful Restart
----------------

BGP graceful restart functionality as defined in
`RFC-4724 <https://tools.ietf.org/html/rfc4724/>`_ defines the mechanisms that
allows BGP speaker to continue to forward data packets along known routes
while the routing protocol information is being restored.


Usually, when BGP on a router restarts, all the BGP peers detect that the
session went down and then came up. This "down/up" transition results in a
"routing flap" and causes BGP route re-computation, generation of BGP routing
updates, and unnecessary churn to the forwarding tables.

The following functionality is provided by graceful restart:

1. The feature allows the restarting router to indicate to the helping peer the
   routes it can preserve in its forwarding plane during control plane restart
   by sending graceful restart capability in the OPEN message sent during
   session establishment.
2. The feature allows helping router to advertise to all other peers the routes
   received from the restarting router which are preserved in the forwarding
   plane of the restarting router during control plane restart.


::



 (R1)-----------------------------------------------------------------(R2)

 1. BGP Graceful Restart Capability exchanged between R1 & R2.

 <--------------------------------------------------------------------->

 2. Kill BGP Process at R1.

 ---------------------------------------------------------------------->

 3. R2 Detects the above BGP Restart & verifies BGP Restarting
   Capability of R1.

 4. Start BGP Process at R1.

 5. Re-establish the BGP session between R1 & R2.

 <--------------------------------------------------------------------->

 6. R2 Send initial route updates, followed by End-Of-Rib.

 <----------------------------------------------------------------------

 7. R1 was waiting for End-Of-Rib from R2 & which has been received
   now.

 8. R1 now runs BGP Best-Path algorithm. Send Initial BGP  Update,
   followed by End-Of Rib

 <--------------------------------------------------------------------->


.. _bgp-GR-preserve-forwarding-state:

BGP-GR Preserve-Forwarding State
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

BGP OPEN message carrying optional capabilities for Graceful Restart has
8 bit “Flags for Address Family” for given AFI and SAFI. This field contains
bit flags relating to routes that were advertised with the given AFI and SAFI.

.. code-block:: frr

   0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |F|   Reserved  |
   +-+-+-+-+-+-+-+-+

The most significant bit is defined as the Forwarding State (F) bit, which
can be used to indicate whether the forwarding state for routes that were
advertised with the given AFI and SAFI has indeed been preserved during the
previous BGP restart. When set (value 1), the bit indicates that the
forwarding state has been preserved.
The remaining bits are reserved and MUST be set to zero by the sender and
ignored by the receiver.

.. clicmd:: bgp graceful-restart preserve-fw-state

FRR gives us the option to enable/disable the "F" flag using this specific
vty command. However, it doesn't have the option to enable/disable
this flag only for specific AFI/SAFI i.e. when this command is used, it
applied to all the supported AFI/SAFI combinations for this peer.

.. _bgp-end-of-rib-message:

End-of-RIB (EOR) message
^^^^^^^^^^^^^^^^^^^^^^^^

An UPDATE message with no reachable Network Layer Reachability  Information
(NLRI) and empty withdrawn NLRI is specified as the End-of-RIB marker that can
be used by a BGP speaker to indicate to its peer the completion of the initial
routing update after the session is established.

For the IPv4 unicast address family, the End-of-RIB marker is an UPDATE message
with the minimum length. For any other address family, it is an UPDATE message
that contains only the MP_UNREACH_NLRI attribute with no withdrawn routes for
that <AFI, SAFI>.

Although the End-of-RIB marker is specified for the purpose of BGP graceful
restart, it is noted that the generation of such a marker upon completion of
the initial update would be useful for routing convergence in general, and thus
the practice is recommended.

.. _bgp-route-selection-deferral-timer:

Route Selection Deferral Timer
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Specifies the time the restarting router defers the route selection process
after restart.

Restarting Router : The usage of route election deferral timer is specified
in https://tools.ietf.org/html/rfc4724#section-4.1

Once the session between the Restarting Speaker and the Receiving Speaker is
re-established, the Restarting Speaker will receive and process BGP messages
from its peers.

However, it MUST defer route selection for an address family until it either.

1. Receives the End-of-RIB marker from all its peers (excluding the ones with
   the "Restart State" bit set in the received capability and excluding the ones
   that do not advertise the graceful restart capability).
2. The Selection_Deferral_Timer timeout.

.. clicmd:: bgp graceful-restart select-defer-time (0-3600)

   This is command, will set deferral time to value specified.


.. clicmd:: bgp graceful-restart rib-stale-time (1-3600)

   This is command, will set the time for which stale routes are kept in RIB.

.. clicmd:: bgp graceful-restart stalepath-time (1-4095)

   This is command, will set the max time (in seconds) to hold onto
   restarting peer's stale paths.

   It also controls Enhanced Route-Refresh timer.

   If this command is configured and the router does not receive a Route-Refresh EoRR
   message, the router removes the stale routes from the BGP table after the timer
   expires. The stale path timer is started when the router receives a Route-Refresh
   BoRR message.

.. _bgp-per-peer-graceful-restart:

BGP Per Peer Graceful Restart
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Ability to enable and disable graceful restart, helper and no GR at all mode
functionality at peer level.

So bgp graceful restart can be enabled at modes  global BGP level or at per
peer level. There are two FSM, one for BGP GR global mode and other for peer
per GR.

Default global mode is helper and default peer per mode is inherit from global.
If per peer mode is configured, the GR mode of this particular peer will
override the global mode.

.. _bgp-GR-global-mode-cmd:

BGP GR Global Mode Commands
^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: bgp graceful-restart

   This command will enable BGP graceful restart ifunctionality at the global
   level.

.. clicmd:: bgp graceful-restart disable

   This command will disable both the functionality graceful restart and helper
   mode.


.. _bgp-GR-peer-mode-cmd:

BGP GR Peer Mode Commands
^^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: neighbor A.B.C.D graceful-restart

   This command will enable BGP graceful restart ifunctionality at the peer
   level.

.. clicmd:: neighbor A.B.C.D graceful-restart-helper

   This command will enable BGP graceful restart helper only functionality
   at the peer level.

.. clicmd:: neighbor A.B.C.D graceful-restart-disable

   This command will disable the entire BGP graceful restart functionality
   at the peer level.


.. _bgp-shutdown:

Administrative Shutdown
-----------------------

.. clicmd:: bgp shutdown [message MSG...]

   Administrative shutdown of all peers of a bgp instance. Drop all BGP peers,
   but preserve their configurations. The peers are notified in accordance with
   `RFC 8203 <https://tools.ietf.org/html/rfc8203/>`_ by sending a
   ``NOTIFICATION`` message with error code ``Cease`` and subcode
   ``Administrative Shutdown`` prior to terminating connections. This global
   shutdown is independent of the neighbor shutdown, meaning that individually
   shut down peers will not be affected by lifting it.

   An optional shutdown message `MSG` can be specified.


.. _bgp-network:

Networks
--------

.. clicmd:: network A.B.C.D/M

   This command adds the announcement network.

   .. code-block:: frr

      router bgp 1
       address-family ipv4 unicast
        network 10.0.0.0/8
       exit-address-family

   This configuration example says that network 10.0.0.0/8 will be
   announced to all neighbors. Some vendors' routers don't advertise
   routes if they aren't present in their IGP routing tables; `bgpd`
   doesn't care about IGP routes when announcing its routes.


.. clicmd:: bgp network import-check

   This configuration modifies the behavior of the network statement.
   If you have this configured the underlying network must exist in
   the rib.  If you have the [no] form configured then BGP will not
   check for the networks existence in the rib.  For versions 7.3 and
   before frr defaults for datacenter were the network must exist,
   traditional did not check for existence.  For versions 7.4 and beyond
   both traditional and datacenter the network must exist.

.. _bgp-ipv6-support:

IPv6 Support
------------

.. clicmd:: neighbor A.B.C.D activate

   This configuration modifies whether to enable an address family for a
   specific neighbor. By default only the IPv4 unicast address family is
   enabled.

   .. code-block:: frr

      router bgp 1
       address-family ipv6 unicast
        neighbor 2001:0DB8::1 activate
        network 2001:0DB8:5009::/64
       exit-address-family

   This configuration example says that network 2001:0DB8:5009::/64 will be
   announced and enables the neighbor 2001:0DB8::1 to receive this announcement.

   By default, only the IPv4 unicast address family is announced to all
   neighbors. Using the 'no bgp default ipv4-unicast' configuration overrides
   this default so that all address families need to be enabled explicitly.

   .. code-block:: frr

      router bgp 1
       no bgp default ipv4-unicast
       neighbor 10.10.10.1 remote-as 2
       neighbor 2001:0DB8::1 remote-as 3
       address-family ipv4 unicast
        neighbor 10.10.10.1 activate
        network 192.168.1.0/24
       exit-address-family
       address-family ipv6 unicast
        neighbor 2001:0DB8::1 activate
        network 2001:0DB8:5009::/64
       exit-address-family

   This configuration demonstrates how the 'no bgp default ipv4-unicast' might
   be used in a setup with two upstreams where each of the upstreams should only
   receive either IPv4 or IPv6 annocuments.

   Using the ``bgp default ipv6-unicast`` configuration, IPv6 unicast
   address family is enabled by default for all new neighbors.


.. _bgp-route-aggregation:

Route Aggregation
-----------------

.. _bgp-route-aggregation-ipv4:

Route Aggregation-IPv4 Address Family
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: aggregate-address A.B.C.D/M

   This command specifies an aggregate address.

.. clicmd:: aggregate-address A.B.C.D/M route-map NAME

   Apply a route-map for an aggregated prefix.

.. clicmd:: aggregate-address A.B.C.D/M origin <egp|igp|incomplete>

   Override ORIGIN for an aggregated prefix.

.. clicmd:: aggregate-address A.B.C.D/M as-set

   This command specifies an aggregate address. Resulting routes include
   AS set.

.. clicmd:: aggregate-address A.B.C.D/M summary-only

   This command specifies an aggregate address. Aggregated routes will
   not be announced.

.. clicmd:: aggregate-address A.B.C.D/M matching-MED-only

   Configure the aggregated address to only be created when the routes MED
   match, otherwise no aggregated route will be created.

.. clicmd:: aggregate-address A.B.C.D/M suppress-map NAME

   Similar to `summary-only`, but will only suppress more specific routes that
   are matched by the selected route-map.


   This configuration example sets up an ``aggregate-address`` under the ipv4
   address-family.

   .. code-block:: frr

      router bgp 1
       address-family ipv4 unicast
        aggregate-address 10.0.0.0/8
        aggregate-address 20.0.0.0/8 as-set
        aggregate-address 40.0.0.0/8 summary-only
        aggregate-address 50.0.0.0/8 route-map aggr-rmap
       exit-address-family


.. _bgp-route-aggregation-ipv6:

Route Aggregation-IPv6 Address Family
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: aggregate-address X:X::X:X/M

   This command specifies an aggregate address.

.. clicmd:: aggregate-address X:X::X:X/M route-map NAME

   Apply a route-map for an aggregated prefix.

.. clicmd:: aggregate-address X:X::X:X/M origin <egp|igp|incomplete>

   Override ORIGIN for an aggregated prefix.

.. clicmd:: aggregate-address X:X::X:X/M as-set

   This command specifies an aggregate address. Resulting routes include
   AS set.

.. clicmd:: aggregate-address X:X::X:X/M summary-only

   This command specifies an aggregate address. Aggregated routes will
   not be announced.

.. clicmd:: aggregate-address X:X::X:X/M matching-MED-only

   Configure the aggregated address to only be created when the routes MED
   match, otherwise no aggregated route will be created.

.. clicmd:: aggregate-address X:X::X:X/M suppress-map NAME

   Similar to `summary-only`, but will only suppress more specific routes that
   are matched by the selected route-map.


   This configuration example sets up an ``aggregate-address`` under the ipv6
   address-family.

   .. code-block:: frr

      router bgp 1
       address-family ipv6 unicast
        aggregate-address 10::0/64
        aggregate-address 20::0/64 as-set
        aggregate-address 40::0/64 summary-only
        aggregate-address 50::0/64 route-map aggr-rmap
       exit-address-family


.. _bgp-redistribute-to-bgp:

Redistribution
--------------

Redistribution configuration should be placed under the ``address-family``
section for the specific AF to redistribute into. Protocol availability for
redistribution is determined by BGP AF; for example, you cannot redistribute
OSPFv3 into ``address-family ipv4 unicast`` as OSPFv3 supports IPv6.

.. clicmd:: redistribute <babel|connected|eigrp|isis|kernel|openfabric|ospf|ospf6|rip|ripng|sharp|static|table> [metric (0-4294967295)] [route-map WORD]

Redistribute routes from other protocols into BGP.

.. clicmd:: redistribute vnc-direct

   Redistribute VNC direct (not via zebra) routes to BGP process.

.. clicmd:: bgp update-delay MAX-DELAY

.. clicmd:: bgp update-delay MAX-DELAY ESTABLISH-WAIT

   This feature is used to enable read-only mode on BGP process restart or when
   a BGP process is cleared using 'clear ip bgp \*'. Note that this command is
   configured at the global level and applies to all bgp instances/vrfs.  It
   cannot be used at the same time as the "update-delay" command described below,
   which is entered in each bgp instance/vrf desired to delay update installation
   and advertisements. The global and per-vrf approaches to defining update-delay
   are mutually exclusive.

   When applicable, read-only mode would begin as soon as the first peer reaches
   Established status and a timer for max-delay seconds is started.  During this
   mode BGP doesn't run any best-path or generate any updates to its peers. This
   mode continues until:

   1. All the configured peers, except the shutdown peers, have sent explicit EOR
      (End-Of-RIB) or an implicit-EOR. The first keep-alive after BGP has reached
      Established is considered an implicit-EOR.
      If the establish-wait optional value is given, then BGP will wait for
      peers to reach established from the beginning of the update-delay till the
      establish-wait period is over, i.e. the minimum set of established peers for
      which EOR is expected would be peers established during the establish-wait
      window, not necessarily all the configured neighbors.
   2. max-delay period is over.

   On hitting any of the above two conditions, BGP resumes the decision process
   and generates updates to its peers.

   Default max-delay is 0, i.e. the feature is off by default.


.. clicmd:: update-delay MAX-DELAY

.. clicmd:: update-delay MAX-DELAY ESTABLISH-WAIT

   This feature is used to enable read-only mode on BGP process restart or when
   a BGP process is cleared using 'clear ip bgp \*'.  Note that this command is
   configured under the specific bgp instance/vrf that the feaure is enabled for.
   It cannot be used at the same time as the global "bgp update-delay" described
   above, which is entered at the global level and applies to all bgp instances.
   The global and per-vrf approaches to defining update-delay are mutually
   exclusive.

   When applicable, read-only mode would begin as soon as the first peer reaches
   Established status and a timer for max-delay seconds is started.  During this
   mode BGP doesn't run any best-path or generate any updates to its peers. This
   mode continues until:

   1. All the configured peers, except the shutdown peers, have sent explicit EOR
      (End-Of-RIB) or an implicit-EOR. The first keep-alive after BGP has reached
      Established is considered an implicit-EOR.
      If the establish-wait optional value is given, then BGP will wait for
      peers to reach established from the beginning of the update-delay till the
      establish-wait period is over, i.e. the minimum set of established peers for
      which EOR is expected would be peers established during the establish-wait
      window, not necessarily all the configured neighbors.
   2. max-delay period is over.

   On hitting any of the above two conditions, BGP resumes the decision process
   and generates updates to its peers.

   Default max-delay is 0, i.e. the feature is off by default.

.. clicmd:: table-map ROUTE-MAP-NAME

   This feature is used to apply a route-map on route updates from BGP to
   Zebra.  All the applicable match operations are allowed, such as match on
   prefix, next-hop, communities, etc. Set operations for this attach-point are
   limited to metric and next-hop only. Any operation of this feature does not
   affect BGPs internal RIB.

   Supported for ipv4 and ipv6 address families. It works on multi-paths as
   well, however, metric setting is based on the best-path only.

.. _bgp-peers:

Peers
-----

.. _bgp-defining-peers:

Defining Peers
^^^^^^^^^^^^^^

.. clicmd:: neighbor PEER remote-as ASN

   Creates a new neighbor whose remote-as is ASN. PEER can be an IPv4 address
   or an IPv6 address or an interface to use for the connection.

   .. code-block:: frr

       router bgp 1
        neighbor 10.0.0.1 remote-as 2

   In this case my router, in AS-1, is trying to peer with AS-2 at 10.0.0.1.

   This command must be the first command used when configuring a neighbor.  If
   the remote-as is not specified, *bgpd* will complain like this: ::

      can't find neighbor 10.0.0.1

.. clicmd:: neighbor PEER remote-as internal

   Create a peer as you would when you specify an ASN, except that if the
   peers ASN is different than mine as specified under the :clicmd:`router bgp ASN`
   command the connection will be denied.

.. clicmd:: neighbor PEER remote-as external

   Create a peer as you would when you specify an ASN, except that if the
   peers ASN is the same as mine as specified under the :clicmd:`router bgp ASN`
   command the connection will be denied.

.. clicmd:: bgp listen range <A.B.C.D/M|X:X::X:X/M> peer-group PGNAME

   Accept connections from any peers in the specified prefix. Configuration
   from the specified peer-group is used to configure these peers.

.. note::

   When using BGP listen ranges, if the associated peer group has TCP MD5
   authentication configured, your kernel must support this on prefixes. On
   Linux, this support was added in kernel version 4.14. If your kernel does
   not support this feature you will get a warning in the log file, and the
   listen range will only accept connections from peers without MD5 configured.

   Additionally, we have observed that when using this option at scale (several
   hundred peers) the kernel may hit its option memory limit. In this situation
   you will see error messages like:

   ``bgpd: sockopt_tcp_signature: setsockopt(23): Cannot allocate memory``

   In this case you need to increase the value of the sysctl
   ``net.core.optmem_max`` to allow the kernel to allocate the necessary option
   memory.

.. clicmd:: bgp listen limit <1-65535>

   Define the maximum number of peers accepted for one BGP instance. This
   limit is set to 100 by default. Increasing this value will really be
   possible if more file descriptors are available in the BGP process. This
   value is defined by the underlying system (ulimit value), and can be
   overriden by `--limit-fds`. More information is available in chapter
   (:ref:`common-invocation-options`).

.. clicmd:: coalesce-time (0-4294967295)

   The time in milliseconds that BGP will delay before deciding what peers
   can be put into an update-group together in order to generate a single
   update for them.  The default time is 1000.

.. _bgp-configuring-peers:

Configuring Peers
^^^^^^^^^^^^^^^^^

.. clicmd:: neighbor PEER shutdown [message MSG...] [rtt (1-65535) [count (1-255)]]

   Shutdown the peer. We can delete the neighbor's configuration by
   ``no neighbor PEER remote-as ASN`` but all configuration of the neighbor
   will be deleted. When you want to preserve the configuration, but want to
   drop the BGP peer, use this syntax.

   Optionally you can specify a shutdown message `MSG`.

   Also, you can specify optionally ``rtt`` in milliseconds to automatically
   shutdown the peer if round-trip-time becomes higher than defined.

   Additional ``count`` parameter is the number of keepalive messages to count
   before shutdown the peer if round-trip-time becomes higher than defined.

.. clicmd:: neighbor PEER disable-connected-check

   Allow peerings between directly connected eBGP peers using loopback
   addresses.

.. clicmd:: neighbor PEER ebgp-multihop

   Specifying ``ebgp-multihop`` allows sessions with eBGP neighbors to
   establish when they are multiple hops away. When the neighbor is not
   directly connected and this knob is not enabled, the session will not
   establish.

   If the peer's IP address is not in the RIB and is reachable via the
   default route, then you have to enable ``ip nht resolve-via-default``.

.. clicmd:: neighbor PEER description ...

   Set description of the peer.

.. clicmd:: neighbor PEER version VERSION

   Set up the neighbor's BGP version. `version` can be `4`, `4+` or `4-`. BGP
   version `4` is the default value used for BGP peering. BGP version `4+`
   means that the neighbor supports Multiprotocol Extensions for BGP-4. BGP
   version `4-` is similar but the neighbor speaks the old Internet-Draft
   revision 00's Multiprotocol Extensions for BGP-4. Some routing software is
   still using this version.

.. clicmd:: neighbor PEER interface IFNAME

   When you connect to a BGP peer over an IPv6 link-local address, you have to
   specify the IFNAME of the interface used for the connection. To specify
   IPv4 session addresses, see the ``neighbor PEER update-source`` command
   below.

   This command is deprecated and may be removed in a future release. Its use
   should be avoided.

.. clicmd:: neighbor PEER interface remote-as <internal|external|ASN>

   Configure an unnumbered BGP peer. ``PEER`` should be an interface name. The
   session will be established via IPv6 link locals. Use ``internal`` for iBGP
   and ``external`` for eBGP sessions, or specify an ASN if you wish.

.. clicmd:: neighbor PEER next-hop-self [force]

   This command specifies an announced route's nexthop as being equivalent to
   the address of the bgp router if it is learned via eBGP. This will also
   bypass third-party next-hops in favor of the local bgp address. If the
   optional keyword ``force`` is specified the modification is done also for
   routes learned via iBGP.

.. clicmd:: neighbor PEER attribute-unchanged [{as-path|next-hop|med}]

   This command specifies attributes to be left unchanged for advertisements
   sent to a peer. Use this to leave the next-hop unchanged in ipv6
   configurations, as the route-map directive to leave the next-hop unchanged
   is only available for ipv4.

.. clicmd:: neighbor PEER update-source <IFNAME|ADDRESS>

   Specify the IPv4 source address to use for the :abbr:`BGP` session to this
   neighbour, may be specified as either an IPv4 address directly or as an
   interface name (in which case the *zebra* daemon MUST be running in order
   for *bgpd* to be able to retrieve interface state).

   .. code-block:: frr

      router bgp 64555
       neighbor foo update-source 192.168.0.1
       neighbor bar update-source lo0


.. clicmd:: neighbor PEER default-originate

   *bgpd*'s default is to not announce the default route (0.0.0.0/0) even if it
   is in routing table. When you want to announce default routes to the peer,
   use this command.

.. clicmd:: neighbor PEER port PORT

.. clicmd:: neighbor PEER password PASSWORD

   Set a MD5 password to be used with the tcp socket that is being used
   to connect to the remote peer.  Please note if you are using this
   command with a large number of peers on linux you should consider
   modifying the `net.core.optmem_max` sysctl to a larger value to
   avoid out of memory errors from the linux kernel.

.. clicmd:: neighbor PEER send-community

.. clicmd:: neighbor PEER weight WEIGHT

   This command specifies a default `weight` value for the neighbor's routes.

.. clicmd:: neighbor PEER maximum-prefix NUMBER [force]

   Sets a maximum number of prefixes we can receive from a given peer. If this
   number is exceeded, the BGP session will be destroyed.

   In practice, it is generally preferable to use a prefix-list to limit what
   prefixes are received from the peer instead of using this knob. Tearing down
   the BGP session when a limit is exceeded is far more destructive than merely
   rejecting undesired prefixes. The prefix-list method is also much more
   granular and offers much smarter matching criterion than number of received
   prefixes, making it more suited to implementing policy.

   If ``force`` is set, then ALL prefixes are counted for maximum instead of
   accepted only. This is useful for cases where an inbound filter is applied,
   but you want maximum-prefix to act on ALL (including filtered) prefixes. This
   option requires `soft-reconfiguration inbound` to be enabled for the peer.

.. clicmd:: neighbor PEER maximum-prefix-out NUMBER

   Sets a maximum number of prefixes we can send to a given peer.

   Since sent prefix count is managed by update-groups, this option
   creates a separate update-group for outgoing updates.

.. clicmd:: neighbor PEER local-as AS-NUMBER [no-prepend] [replace-as]

   Specify an alternate AS for this BGP process when interacting with the
   specified peer. With no modifiers, the specified local-as is prepended to
   the received AS_PATH when receiving routing updates from the peer, and
   prepended to the outgoing AS_PATH (after the process local AS) when
   transmitting local routes to the peer.

   If the no-prepend attribute is specified, then the supplied local-as is not
   prepended to the received AS_PATH.

   If the replace-as attribute is specified, then only the supplied local-as is
   prepended to the AS_PATH when transmitting local-route updates to this peer.

   Note that replace-as can only be specified if no-prepend is.

   This command is only allowed for eBGP peers.

.. clicmd:: neighbor <A.B.C.D|X:X::X:X|WORD> as-override

   Override AS number of the originating router with the local AS number.

   Usually this configuration is used in PEs (Provider Edge) to replace
   the incoming customer AS number so the connected CE (Customer Edge)
   can use the same AS number as the other customer sites. This allows
   customers of the provider network to use the same AS number across
   their sites.

   This command is only allowed for eBGP peers.

.. clicmd:: neighbor <A.B.C.D|X:X::X:X|WORD> allowas-in [<(1-10)|origin>]

   Accept incoming routes with AS path containing AS number with the same value
   as the current system AS.

   This is used when you want to use the same AS number in your sites, but you
   can't connect them directly. This is an alternative to
   `neighbor WORD as-override`.

   The parameter `(1-10)` configures the amount of accepted occurences of the
   system AS number in AS path.

   The parameter `origin` configures BGP to only accept routes originated with
   the same AS number as the system.

   This command is only allowed for eBGP peers.

.. clicmd:: neighbor <A.B.C.D|X:X::X:X|WORD> addpath-tx-all-paths

   Configure BGP to send all known paths to neighbor in order to preserve multi
   path capabilities inside a network.

.. clicmd:: neighbor <A.B.C.D|X:X::X:X|WORD> addpath-tx-bestpath-per-AS

   Configure BGP to send best known paths to neighbor in order to preserve multi
   path capabilities inside a network.

.. clicmd:: neighbor PEER ttl-security hops NUMBER

   This command enforces Generalized TTL Security Mechanism (GTSM), as
   specified in RFC 5082. With this command, only neighbors that are the
   specified number of hops away will be allowed to become neighbors. This
   command is mutually exclusive with *ebgp-multihop*.

.. clicmd:: neighbor PEER capability extended-nexthop

   Allow bgp to negotiate the extended-nexthop capability with it's peer.
   If you are peering over a v6 LL address then this capability is turned
   on automatically.  If you are peering over a v6 Global Address then
   turning on this command will allow BGP to install v4 routes with
   v6 nexthops if you do not have v4 configured on interfaces.

.. clicmd:: bgp fast-external-failover

   This command causes bgp to not take down ebgp peers immediately
   when a link flaps.  `bgp fast-external-failover` is the default
   and will not be displayed as part of a `show run`.  The no form
   of the command turns off this ability.

.. clicmd:: bgp default ipv4-unicast

   This command allows the user to specify that the IPv4 Unicast address
   family is turned on by default or not.  This command defaults to on
   and is not displayed.
   The `no bgp default ipv4-unicast` form of the command is displayed.

.. clicmd:: bgp default ipv4-multicast

   This command allows the user to specify that the IPv4 Multicast address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv4-multicast` form of the command is displayed.

.. clicmd:: bgp default ipv4-vpn

   This command allows the user to specify that the IPv4 MPLS VPN address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv4-vpn` form of the command is displayed.

.. clicmd:: bgp default ipv4-flowspec

   This command allows the user to specify that the IPv4 Flowspec address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv4-flowspec` form of the command is displayed.

.. clicmd:: bgp default ipv6-unicast

   This command allows the user to specify that the IPv6 Unicast address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv6-unicast` form of the command is displayed.

.. clicmd:: bgp default ipv6-multicast

   This command allows the user to specify that the IPv6 Multicast address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv6-multicast` form of the command is displayed.

.. clicmd:: bgp default ipv6-vpn

   This command allows the user to specify that the IPv6 MPLS VPN address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv6-vpn` form of the command is displayed.

.. clicmd:: bgp default ipv6-flowspec

   This command allows the user to specify that the IPv6 Flowspec address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default ipv6-flowspec` form of the command is displayed.

.. clicmd:: bgp default l2vpn-evpn

   This command allows the user to specify that the L2VPN EVPN address
   family is turned on by default or not.  This command defaults to off
   and is not displayed.
   The `bgp default l2vpn-evpn` form of the command is displayed.

.. clicmd:: bgp default show-hostname

   This command shows the hostname of the peer in certain BGP commands
   outputs. It's easier to troubleshoot if you have a number of BGP peers.

.. clicmd:: bgp default show-nexthop-hostname

   This command shows the hostname of the next-hop in certain BGP commands
   outputs. It's easier to troubleshoot if you have a number of BGP peers
   and a number of routes to check.

.. clicmd:: neighbor PEER advertisement-interval (0-600)

   Setup the minimum route advertisement interval(mrai) for the
   peer in question.  This number is between 0 and 600 seconds,
   with the default advertisement interval being 0.

.. clicmd:: neighbor PEER timers (0-65535) (0-65535)

   Set keepalive and hold timers for a neighbor. The first value is keepalive
   and the second is hold time.

.. clicmd:: neighbor PEER connect (1-65535)

   Set connect timer for a neighbor. The connect timer controls how long BGP
   waits between connection attempts to a neighbor.

.. clicmd:: neighbor PEER timers delayopen (1-240)

   This command allows the user enable the
   `RFC 4271 <https://tools.ietf.org/html/rfc4271/>` DelayOpenTimer with the
   specified interval or disable it with the negating command for the peer. By
   default, the DelayOpenTimer is disabled. The timer interval may be set to a
   duration of 1 to 240 seconds.

Displaying Information about Peers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: show bgp <afi> <safi> neighbors WORD bestpath-routes [json] [wide]

   For the given neighbor, WORD, that is specified list the routes selected
   by BGP as having the best path.

.. _bgp-peer-filtering:

Peer Filtering
^^^^^^^^^^^^^^

.. clicmd:: neighbor PEER distribute-list NAME [in|out]

   This command specifies a distribute-list for the peer. `direct` is
   ``in`` or ``out``.

.. clicmd:: neighbor PEER prefix-list NAME [in|out]

.. clicmd:: neighbor PEER filter-list NAME [in|out]

.. clicmd:: neighbor PEER route-map NAME [in|out]

   Apply a route-map on the neighbor. `direct` must be `in` or `out`.

.. clicmd:: bgp route-reflector allow-outbound-policy

   By default, attribute modification via route-map policy out is not reflected
   on reflected routes. This option allows the modifications to be reflected as
   well. Once enabled, it affects all reflected routes.

.. clicmd:: neighbor PEER sender-as-path-loop-detection

   Enable the detection of sender side AS path loops and filter the
   bad routes before they are sent.

   This setting is disabled by default.

.. _bgp-peer-group:

Peer Groups
^^^^^^^^^^^

Peer groups are used to help improve scaling by generating the same
update information to all members of a peer group. Note that this means
that the routes generated by a member of a peer group will be sent back
to that originating peer with the originator identifier attribute set to
indicated the originating peer.  All peers not associated with a
specific peer group are treated as belonging to a default peer group,
and will share updates.

.. clicmd:: neighbor WORD peer-group

   This command defines a new peer group.

.. clicmd:: neighbor PEER peer-group PGNAME

   This command bind specific peer to peer group WORD.

.. clicmd:: neighbor PEER solo

   This command is used to indicate that routes advertised by the peer
   should not be reflected back to the peer.  This command only is only
   meaningful when there is a single peer defined in the peer-group.

.. clicmd:: show [ip] bgp peer-group [json]

   This command displays configured BGP peer-groups.

      .. code-block:: frr

         exit1-debian-9# show bgp peer-group

         BGP peer-group test1, remote AS 65001
         Peer-group type is external
         Configured address-families: IPv4 Unicast; IPv6 Unicast;
         1 IPv4 listen range(s)
            192.168.100.0/24
         2 IPv6 listen range(s)
            2001:db8:1::/64
            2001:db8:2::/64
         Peer-group members:
            192.168.200.1  Active
            2001:db8::1  Active

         BGP peer-group test2
         Peer-group type is external
         Configured address-families: IPv4 Unicast;

   Optional ``json`` parameter is used to display JSON output.

      .. code-block:: frr

         {
           "test1":{
             "remoteAs":65001,
             "type":"external",
             "addressFamiliesConfigured":[
               "IPv4 Unicast",
               "IPv6 Unicast"
             ],
             "dynamicRanges":{
               "IPv4":{
                 "count":1,
                 "ranges":[
                   "192.168.100.0\/24"
                 ]
               },
               "IPv6":{
                 "count":2,
                 "ranges":[
                   "2001:db8:1::\/64",
                   "2001:db8:2::\/64"
                 ]
               }
             },
             "members":{
               "192.168.200.1":{
                 "status":"Active"
               },
               "2001:db8::1":{
                 "status":"Active"
               }
             }
           },
           "test2":{
              "type":"external",
              "addressFamiliesConfigured":[
                "IPv4 Unicast"
              ]
           }
         }

Capability Negotiation
^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: neighbor PEER strict-capability-match


   Strictly compares remote capabilities and local capabilities. If
   capabilities are different, send Unsupported Capability error then reset
   connection.

   You may want to disable sending Capability Negotiation OPEN message optional
   parameter to the peer when remote peer does not implement Capability
   Negotiation. Please use *dont-capability-negotiate* command to disable the
   feature.

.. clicmd:: neighbor PEER dont-capability-negotiate

   Suppress sending Capability Negotiation as OPEN message optional parameter
   to the peer. This command only affects the peer is configured other than
   IPv4 unicast configuration.

   When remote peer does not have capability negotiation feature, remote peer
   will not send any capabilities at all. In that case, bgp configures the peer
   with configured capabilities.

   You may prefer locally configured capabilities more than the negotiated
   capabilities even though remote peer sends capabilities. If the peer is
   configured by *override-capability*, *bgpd* ignores received capabilities
   then override negotiated capabilities with configured values.

   Additionally the operator should be reminded that this feature fundamentally
   disables the ability to use widely deployed BGP features.  BGP unnumbered,
   hostname support, AS4, Addpath, Route Refresh, ORF, Dynamic Capabilities,
   and graceful restart.

.. clicmd:: neighbor PEER override-capability


   Override the result of Capability Negotiation with local configuration.
   Ignore remote peer's capability value.

.. _bgp-as-path-access-lists:

AS Path Access Lists
--------------------

AS path access list is user defined AS path.

.. clicmd:: bgp as-path access-list WORD [seq (0-4294967295)] permit|deny LINE

   This command defines a new AS path access list.

.. clicmd:: show bgp as-path-access-list [json]

   Display all BGP AS Path access lists.

   If the ``json`` option is specified, output is displayed in JSON format.

.. clicmd:: show bgp as-path-access-list WORD [json]

   Display the specified BGP AS Path access list.

   If the ``json`` option is specified, output is displayed in JSON format.

.. _bgp-bogon-filter-example:

Bogon ASN filter policy configuration example
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: frr

   bgp as-path access-list 99 permit _0_
   bgp as-path access-list 99 permit _23456_
   bgp as-path access-list 99 permit _1310[0-6][0-9]_|_13107[0-1]_
   bgp as-path access-list 99 seq 20 permit ^65

.. _bgp-using-as-path-in-route-map:

Using AS Path in Route Map
--------------------------

.. clicmd:: match as-path WORD

   For a given as-path, WORD, match it on the BGP as-path given for the prefix
   and if it matches do normal route-map actions.  The no form of the command
   removes this match from the route-map.

.. clicmd:: set as-path prepend AS-PATH

   Prepend the given string of AS numbers to the AS_PATH of the BGP path's NLRI.
   The no form of this command removes this set operation from the route-map.

.. clicmd:: set as-path prepend last-as NUM

   Prepend the existing last AS number (the leftmost ASN) to the AS_PATH.
   The no form of this command removes this set operation from the route-map.

.. _bgp-communities-attribute:

Communities Attribute
---------------------

The BGP communities attribute is widely used for implementing policy routing.
Network operators can manipulate BGP communities attribute based on their
network policy. BGP communities attribute is defined in :rfc:`1997` and
:rfc:`1998`. It is an optional transitive attribute, therefore local policy can
travel through different autonomous system.

The communities attribute is a set of communities values. Each community value
is 4 octet long. The following format is used to define the community value.

``AS:VAL``
   This format represents 4 octet communities value. ``AS`` is high order 2
   octet in digit format. ``VAL`` is low order 2 octet in digit format. This
   format is useful to define AS oriented policy value. For example,
   ``7675:80`` can be used when AS 7675 wants to pass local policy value 80 to
   neighboring peer.

``internet``
   ``internet`` represents well-known communities value 0.

``graceful-shutdown``
   ``graceful-shutdown`` represents well-known communities value
   ``GRACEFUL_SHUTDOWN`` ``0xFFFF0000`` ``65535:0``. :rfc:`8326` implements
   the purpose Graceful BGP Session Shutdown to reduce the amount of
   lost traffic when taking BGP sessions down for maintenance. The use
   of the community needs to be supported from your peers side to
   actually have any effect.

``accept-own``
   ``accept-own`` represents well-known communities value ``ACCEPT_OWN``
   ``0xFFFF0001`` ``65535:1``. :rfc:`7611` implements a way to signal
   to a router to accept routes with a local nexthop address. This
   can be the case when doing policing and having traffic having a
   nexthop located in another VRF but still local interface to the
   router. It is recommended to read the RFC for full details.

``route-filter-translated-v4``
   ``route-filter-translated-v4`` represents well-known communities value
   ``ROUTE_FILTER_TRANSLATED_v4`` ``0xFFFF0002`` ``65535:2``.

``route-filter-v4``
   ``route-filter-v4`` represents well-known communities value
   ``ROUTE_FILTER_v4`` ``0xFFFF0003`` ``65535:3``.

``route-filter-translated-v6``
   ``route-filter-translated-v6`` represents well-known communities value
   ``ROUTE_FILTER_TRANSLATED_v6`` ``0xFFFF0004`` ``65535:4``.

``route-filter-v6``
   ``route-filter-v6`` represents well-known communities value
   ``ROUTE_FILTER_v6`` ``0xFFFF0005`` ``65535:5``.

``llgr-stale``
   ``llgr-stale`` represents well-known communities value ``LLGR_STALE``
   ``0xFFFF0006`` ``65535:6``.
   Assigned and intended only for use with routers supporting the
   Long-lived Graceful Restart Capability  as described in
   [Draft-IETF-uttaro-idr-bgp-persistence]_.
   Routers receiving routes with this community may (depending on
   implementation) choose allow to reject or modify routes on the
   presence or absence of this community.

``no-llgr``
   ``no-llgr`` represents well-known communities value ``NO_LLGR``
   ``0xFFFF0007`` ``65535:7``.
   Assigned and intended only for use with routers supporting the
   Long-lived Graceful Restart Capability  as described in
   [Draft-IETF-uttaro-idr-bgp-persistence]_.
   Routers receiving routes with this community may (depending on
   implementation) choose allow to reject or modify routes on the
   presence or absence of this community.

``accept-own-nexthop``
   ``accept-own-nexthop`` represents well-known communities value
   ``accept-own-nexthop`` ``0xFFFF0008`` ``65535:8``.
   [Draft-IETF-agrewal-idr-accept-own-nexthop]_ describes
   how to tag and label VPN routes to be able to send traffic between VRFs
   via an internal layer 2 domain on the same PE device. Refer to
   [Draft-IETF-agrewal-idr-accept-own-nexthop]_ for full details.

``blackhole``
   ``blackhole`` represents well-known communities value ``BLACKHOLE``
   ``0xFFFF029A`` ``65535:666``. :rfc:`7999` documents sending prefixes to
   EBGP peers and upstream for the purpose of blackholing traffic.
   Prefixes tagged with the this community should normally not be
   re-advertised from neighbors of the originating network. Upon receiving
   ``BLACKHOLE`` community from a BGP speaker, ``NO_ADVERTISE`` community
   is added automatically.

``no-export``
   ``no-export`` represents well-known communities value ``NO_EXPORT``
   ``0xFFFFFF01``. All routes carry this value must not be advertised to
   outside a BGP confederation boundary. If neighboring BGP peer is part of BGP
   confederation, the peer is considered as inside a BGP confederation
   boundary, so the route will be announced to the peer.

``no-advertise``
   ``no-advertise`` represents well-known communities value ``NO_ADVERTISE``
   ``0xFFFFFF02``. All routes carry this value must not be advertise to other
   BGP peers.

``local-AS``
   ``local-AS`` represents well-known communities value ``NO_EXPORT_SUBCONFED``
   ``0xFFFFFF03``. All routes carry this value must not be advertised to
   external BGP peers. Even if the neighboring router is part of confederation,
   it is considered as external BGP peer, so the route will not be announced to
   the peer.

``no-peer``
   ``no-peer`` represents well-known communities value ``NOPEER``
   ``0xFFFFFF04``  ``65535:65284``. :rfc:`3765` is used to communicate to
   another network how the originating network want the prefix propagated.

When the communities attribute is received duplicate community values in the
attribute are ignored and value is sorted in numerical order.

.. [Draft-IETF-uttaro-idr-bgp-persistence] <https://tools.ietf.org/id/draft-uttaro-idr-bgp-persistence-04.txt>
.. [Draft-IETF-agrewal-idr-accept-own-nexthop] <https://tools.ietf.org/id/draft-agrewal-idr-accept-own-nexthop-00.txt>

.. _bgp-community-lists:

Community Lists
^^^^^^^^^^^^^^^
Community lists are user defined lists of community attribute values. These
lists can be used for matching or manipulating the communities attribute in
UPDATE messages.

There are two types of community list:

standard
   This type accepts an explicit value for the attribute.

expanded
   This type accepts a regular expression. Because the regex must be
   interpreted on each use expanded community lists are slower than standard
   lists.

.. clicmd:: bgp community-list standard NAME permit|deny COMMUNITY

   This command defines a new standard community list. ``COMMUNITY`` is
   communities value. The ``COMMUNITY`` is compiled into community structure.
   We can define multiple community list under same name. In that case match
   will happen user defined order. Once the community list matches to
   communities attribute in BGP updates it return permit or deny by the
   community list definition. When there is no matched entry, deny will be
   returned. When ``COMMUNITY`` is empty it matches to any routes.

.. clicmd:: bgp community-list expanded NAME permit|deny COMMUNITY

   This command defines a new expanded community list. ``COMMUNITY`` is a
   string expression of communities attribute. ``COMMUNITY`` can be a regular
   expression (:ref:`bgp-regular-expressions`) to match the communities
   attribute in BGP updates. The expanded community is only used to filter,
   not `set` actions.

.. deprecated:: 5.0
   It is recommended to use the more explicit versions of this command.

.. clicmd:: bgp community-list NAME permit|deny COMMUNITY

   When the community list type is not specified, the community list type is
   automatically detected. If ``COMMUNITY`` can be compiled into communities
   attribute, the community list is defined as a standard community list.
   Otherwise it is defined as an expanded community list. This feature is left
   for backward compatibility. Use of this feature is not recommended.

   Note that all community lists share the same namespace, so it's not
   necessary to specify ``standard`` or ``expanded``; these modifiers are
   purely aesthetic.

.. clicmd:: show bgp community-list [NAME detail]

   Displays community list information. When ``NAME`` is specified the
   specified community list's information is shown.

   ::

       # show bgp community-list
       Named Community standard list CLIST
       permit 7675:80 7675:100 no-export
       deny internet
         Named Community expanded list EXPAND
       permit :

         # show bgp community-list CLIST detail
         Named Community standard list CLIST
       permit 7675:80 7675:100 no-export
       deny internet


.. _bgp-numbered-community-lists:

Numbered Community Lists
^^^^^^^^^^^^^^^^^^^^^^^^

When number is used for BGP community list name, the number has
special meanings. Community list number in the range from 1 and 99 is
standard community list. Community list number in the range from 100
to 199 is expanded community list. These community lists are called
as numbered community lists. On the other hand normal community lists
is called as named community lists.

.. clicmd:: bgp community-list (1-99) permit|deny COMMUNITY

   This command defines a new community list. The argument to (1-99) defines
   the list identifier.

.. clicmd:: bgp community-list (100-199) permit|deny COMMUNITY

   This command defines a new expanded community list. The argument to
   (100-199) defines the list identifier.

.. _bgp-community-alias:

Community alias
^^^^^^^^^^^^^^^

BGP community aliases are useful to quickly identify what communities are set
for a specific prefix in a human-readable format. Especially handy for a huge
amount of communities. Accurately defined aliases can help you faster spot
things on the wire.

.. clicmd:: bgp community alias NAME ALIAS

   This command creates an alias name for a community that will be used
   later in various CLI outputs in a human-readable format.

   .. code-block:: frr

      ~# vtysh -c 'show run' | grep 'bgp community alias'
      bgp community alias 65001:14 community-1
      bgp community alias 65001:123:1 lcommunity-1

      ~# vtysh -c 'show ip bgp 172.16.16.1/32'
      BGP routing table entry for 172.16.16.1/32, version 21
      Paths: (2 available, best #2, table default)
        Advertised to non peer-group peers:
        65030
          192.168.0.2 from 192.168.0.2 (172.16.16.1)
            Origin incomplete, metric 0, valid, external, best (Neighbor IP)
            Community: 65001:12 65001:13 community-1 65001:65534
            Large Community: lcommunity-1 65001:123:2
            Last update: Fri Apr 16 12:51:27 2021

.. clicmd:: show bgp [afi] [safi] [all] alias WORD [wide|json]

   Display prefixes with matching BGP community alias.

.. _bgp-using-communities-in-route-map:

Using Communities in Route Maps
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In :ref:`route-map` we can match on or set the BGP communities attribute. Using
this feature network operator can implement their network policy based on BGP
communities attribute.

The following commands can be used in route maps:

.. clicmd:: match alias WORD

   This command performs match to BGP updates using community alias WORD. When
   the one of BGP communities value match to the one of community alias value in
   community alias, it is match.

.. clicmd:: match community WORD exact-match [exact-match]

   This command perform match to BGP updates using community list WORD. When
   the one of BGP communities value match to the one of communities value in
   community list, it is match. When `exact-match` keyword is specified, match
   happen only when BGP updates have completely same communities value
   specified in the community list.

.. clicmd:: set community <none|COMMUNITY> additive

   This command sets the community value in BGP updates.  If the attribute is
   already configured, the newly provided value replaces the old one unless the
   ``additive`` keyword is specified, in which case the new value is appended
   to the existing value.

   If ``none`` is specified as the community value, the communities attribute
   is not sent.

   It is not possible to set an expanded community list.

.. clicmd:: set comm-list WORD delete

   This command remove communities value from BGP communities attribute.  The
   ``word`` is community list name. When BGP route's communities value matches
   to the community list ``word``, the communities value is removed. When all
   of communities value is removed eventually, the BGP update's communities
   attribute is completely removed.

.. _bgp-communities-example:

Example Configuration
^^^^^^^^^^^^^^^^^^^^^

The following configuration is exemplary of the most typical usage of BGP
communities attribute. In the example, AS 7675 provides an upstream Internet
connection to AS 100. When the following configuration exists in AS 7675, the
network operator of AS 100 can set local preference in AS 7675 network by
setting BGP communities attribute to the updates.

.. code-block:: frr

   router bgp 7675
    neighbor 192.168.0.1 remote-as 100
    address-family ipv4 unicast
     neighbor 192.168.0.1 route-map RMAP in
    exit-address-family
   !
   bgp community-list 70 permit 7675:70
   bgp community-list 70 deny
   bgp community-list 80 permit 7675:80
   bgp community-list 80 deny
   bgp community-list 90 permit 7675:90
   bgp community-list 90 deny
   !
   route-map RMAP permit 10
    match community 70
    set local-preference 70
   !
   route-map RMAP permit 20
    match community 80
    set local-preference 80
   !
   route-map RMAP permit 30
    match community 90
    set local-preference 90


The following configuration announces ``10.0.0.0/8`` from AS 100 to AS 7675.
The route has communities value ``7675:80`` so when above configuration exists
in AS 7675, the announced routes' local preference value will be set to 80.

.. code-block:: frr

   router bgp 100
    network 10.0.0.0/8
    neighbor 192.168.0.2 remote-as 7675
    address-family ipv4 unicast
     neighbor 192.168.0.2 route-map RMAP out
    exit-address-family
   !
   ip prefix-list PLIST permit 10.0.0.0/8
   !
   route-map RMAP permit 10
    match ip address prefix-list PLIST
    set community 7675:80


The following configuration is an example of BGP route filtering using
communities attribute. This configuration only permit BGP routes which has BGP
communities value ``0:80`` or ``0:90``. The network operator can set special
internal communities value at BGP border router, then limit the BGP route
announcements into the internal network.

.. code-block:: frr

   router bgp 7675
    neighbor 192.168.0.1 remote-as 100
    address-family ipv4 unicast
     neighbor 192.168.0.1 route-map RMAP in
    exit-address-family
   !
   bgp community-list 1 permit 0:80 0:90
   !
   route-map RMAP permit in
    match community 1


The following example filters BGP routes which have a community value of
``1:1``. When there is no match community-list returns ``deny``. To avoid
filtering all routes, a ``permit`` line is set at the end of the
community-list.

.. code-block:: frr

   router bgp 7675
    neighbor 192.168.0.1 remote-as 100
    address-family ipv4 unicast
     neighbor 192.168.0.1 route-map RMAP in
    exit-address-family
   !
   bgp community-list standard FILTER deny 1:1
   bgp community-list standard FILTER permit
   !
   route-map RMAP permit 10
    match community FILTER


The communities value keyword ``internet`` has special meanings in standard
community lists. In the below example ``internet`` matches all BGP routes even
if the route does not have communities attribute at all. So community list
``INTERNET`` is the same as ``FILTER`` in the previous example.

.. code-block:: frr

   bgp community-list standard INTERNET deny 1:1
   bgp community-list standard INTERNET permit internet


The following configuration is an example of communities value deletion.  With
this configuration the community values ``100:1`` and ``100:2`` are removed
from BGP updates. For communities value deletion, only ``permit``
community-list is used. ``deny`` community-list is ignored.

.. code-block:: frr

   router bgp 7675
    neighbor 192.168.0.1 remote-as 100
    address-family ipv4 unicast
     neighbor 192.168.0.1 route-map RMAP in
    exit-address-family
   !
   bgp community-list standard DEL permit 100:1 100:2
   !
   route-map RMAP permit 10
    set comm-list DEL delete


.. _bgp-extended-communities-attribute:

Extended Communities Attribute
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

BGP extended communities attribute is introduced with MPLS VPN/BGP technology.
MPLS VPN/BGP expands capability of network infrastructure to provide VPN
functionality. At the same time it requires a new framework for policy routing.
With BGP Extended Communities Attribute we can use Route Target or Site of
Origin for implementing network policy for MPLS VPN/BGP.

BGP Extended Communities Attribute is similar to BGP Communities Attribute. It
is an optional transitive attribute. BGP Extended Communities Attribute can
carry multiple Extended Community value.  Each Extended Community value is
eight octet length.

BGP Extended Communities Attribute provides an extended range compared with BGP
Communities Attribute. Adding to that there is a type field in each value to
provides community space structure.

There are two format to define Extended Community value. One is AS based format
the other is IP address based format.

``AS:VAL``
   This is a format to define AS based Extended Community value.  ``AS`` part
   is 2 octets Global Administrator subfield in Extended Community value.
   ``VAL`` part is 4 octets Local Administrator subfield. ``7675:100``
   represents AS 7675 policy value 100.

``IP-Address:VAL``
   This is a format to define IP address based Extended Community value.
   ``IP-Address`` part is 4 octets Global Administrator subfield.  ``VAL`` part
   is 2 octets Local Administrator subfield.

.. _bgp-extended-community-lists:

Extended Community Lists
^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: bgp extcommunity-list standard NAME permit|deny EXTCOMMUNITY

   This command defines a new standard extcommunity-list. `extcommunity` is
   extended communities value. The `extcommunity` is compiled into extended
   community structure. We can define multiple extcommunity-list under same
   name. In that case match will happen user defined order. Once the
   extcommunity-list matches to extended communities attribute in BGP updates
   it return permit or deny based upon the extcommunity-list definition. When
   there is no matched entry, deny will be returned. When `extcommunity` is
   empty it matches to any routes.

.. clicmd:: bgp extcommunity-list expanded NAME permit|deny LINE

   This command defines a new expanded extcommunity-list. `line` is a string
   expression of extended communities attribute. `line` can be a regular
   expression (:ref:`bgp-regular-expressions`) to match an extended communities
   attribute in BGP updates.

   Note that all extended community lists shares a single name space, so it's
   not necessary to specify their type when creating or destroying them.

.. clicmd:: show bgp extcommunity-list [NAME detail]

   This command displays current extcommunity-list information. When `name` is
   specified the community list's information is shown.


.. _bgp-extended-communities-in-route-map:

BGP Extended Communities in Route Map
"""""""""""""""""""""""""""""""""""""

.. clicmd:: match extcommunity WORD

.. clicmd:: set extcommunity rt EXTCOMMUNITY

   This command set Route Target value.

.. clicmd:: set extcommunity soo EXTCOMMUNITY

   This command set Site of Origin value.

.. clicmd:: set extcommunity bandwidth <(1-25600) | cumulative | num-multipaths> [non-transitive]

   This command sets the BGP link-bandwidth extended community for the prefix
   (best path) for which it is applied. The link-bandwidth can be specified as
   an ``explicit value`` (specified in Mbps), or the router can be told to use
   the ``cumulative bandwidth`` of all multipaths for the prefix or to compute
   it based on the ``number of multipaths``.  The link bandwidth extended
   community is encoded as ``transitive`` unless the set command explicitly
   configures it as ``non-transitive``.

.. seealso:: :ref:`wecmp_linkbw`

Note that the extended expanded community is only used for `match` rule, not for
`set` actions.

.. _bgp-large-communities-attribute:

Large Communities Attribute
^^^^^^^^^^^^^^^^^^^^^^^^^^^

The BGP Large Communities attribute was introduced in Feb 2017 with
:rfc:`8092`.

The BGP Large Communities Attribute is similar to the BGP Communities Attribute
except that it has 3 components instead of two and each of which are 4 octets
in length. Large Communities bring additional functionality and convenience
over traditional communities, specifically the fact that the ``GLOBAL`` part
below is now 4 octets wide allowing seamless use in networks using 4-byte ASNs.

``GLOBAL:LOCAL1:LOCAL2``
   This is the format to define Large Community values. Referencing :rfc:`8195`
   the values are commonly referred to as follows:

   - The ``GLOBAL`` part is a 4 octet Global Administrator field, commonly used
     as the operators AS number.
   - The ``LOCAL1`` part is a 4 octet Local Data Part 1 subfield referred to as
     a function.
   - The ``LOCAL2`` part is a 4 octet Local Data Part 2 field and referred to
     as the parameter subfield.

   As an example, ``65551:1:10`` represents AS 65551 function 1 and parameter
   10. The referenced RFC above gives some guidelines on recommended usage.

.. _bgp-large-community-lists:

Large Community Lists
"""""""""""""""""""""

Two types of large community lists are supported, namely `standard` and
`expanded`.

.. clicmd:: bgp large-community-list standard NAME permit|deny LARGE-COMMUNITY

   This command defines a new standard large-community-list.  `large-community`
   is the Large Community value. We can add multiple large communities under
   same name. In that case the match will happen in the user defined order.
   Once the large-community-list matches the Large Communities attribute in BGP
   updates it will return permit or deny based upon the large-community-list
   definition. When there is no matched entry, a deny will be returned. When
   `large-community` is empty it matches any routes.

.. clicmd:: bgp large-community-list expanded NAME permit|deny LINE

   This command defines a new expanded large-community-list. Where `line` is a
   string matching expression, it will be compared to the entire Large
   Communities attribute as a string, with each large-community in order from
   lowest to highest.  `line` can also be a regular expression which matches
   this Large Community attribute.

   Note that all community lists share the same namespace, so it's not
   necessary to specify ``standard`` or ``expanded``; these modifiers are
   purely aesthetic.

.. clicmd:: show bgp large-community-list

.. clicmd:: show bgp large-community-list NAME detail

   This command display current large-community-list information. When
   `name` is specified the community list information is shown.

.. clicmd:: show ip bgp large-community-info

   This command displays the current large communities in use.

.. _bgp-large-communities-in-route-map:

Large Communities in Route Map
""""""""""""""""""""""""""""""

.. clicmd:: match large-community LINE [exact-match]

   Where `line` can be a simple string to match, or a regular expression. It
   is very important to note that this match occurs on the entire
   large-community string as a whole, where each large-community is ordered
   from lowest to highest. When `exact-match` keyword is specified, match
   happen only when BGP updates have completely same large communities value
   specified in the large community list.

.. clicmd:: set large-community LARGE-COMMUNITY

.. clicmd:: set large-community LARGE-COMMUNITY LARGE-COMMUNITY

.. clicmd:: set large-community LARGE-COMMUNITY additive

   These commands are used for setting large-community values. The first
   command will overwrite any large-communities currently present.
   The second specifies two large-communities, which overwrites the current
   large-community list. The third will add a large-community value without
   overwriting other values. Multiple large-community values can be specified.

Note that the large expanded community is only used for `match` rule, not for
`set` actions.

.. _bgp-l3vpn-vrfs:

L3VPN VRFs
----------

*bgpd* supports :abbr:`L3VPN (Layer 3 Virtual Private Networks)` :abbr:`VRFs
(Virtual Routing and Forwarding)` for IPv4 :rfc:`4364` and IPv6 :rfc:`4659`.
L3VPN routes, and their associated VRF MPLS labels, can be distributed to VPN
SAFI neighbors in the *default*, i.e., non VRF, BGP instance. VRF MPLS labels
are reached using *core* MPLS labels which are distributed using LDP or BGP
labeled unicast.  *bgpd* also supports inter-VRF route leaking.


.. _bgp-vrf-route-leaking:

VRF Route Leaking
-----------------

BGP routes may be leaked (i.e. copied) between a unicast VRF RIB and the VPN
SAFI RIB of the default VRF for use in MPLS-based L3VPNs. Unicast routes may
also be leaked between any VRFs (including the unicast RIB of the default BGP
instanced). A shortcut syntax is also available for specifying leaking from one
VRF to another VRF using the default instance's VPN RIB as the intemediary. A
common application of the VRF-VRF feature is to connect a customer's private
routing domain to a provider's VPN service. Leaking is configured from the
point of view of an individual VRF: ``import`` refers to routes leaked from VPN
to a unicast VRF, whereas ``export`` refers to routes leaked from a unicast VRF
to VPN.

Required parameters
^^^^^^^^^^^^^^^^^^^

Routes exported from a unicast VRF to the VPN RIB must be augmented by two
parameters:

- an :abbr:`RD (Route Distinguisher)`
- an :abbr:`RTLIST (Route-target List)`

Configuration for these exported routes must, at a minimum, specify these two
parameters.

Routes imported from the VPN RIB to a unicast VRF are selected according to
their RTLISTs.  Routes whose RTLIST contains at least one route-target in
common with the configured import RTLIST are leaked.  Configuration for these
imported routes must specify an RTLIST to be matched.

The RD, which carries no semantic value, is intended to make the route unique
in the VPN RIB among all routes of its prefix that originate from all the
customers and sites that are attached to the provider's VPN service.
Accordingly, each site of each customer is typically assigned an RD that is
unique across the entire provider network.

The RTLIST is a set of route-target extended community values whose purpose is
to specify route-leaking policy. Typically, a customer is assigned a single
route-target value for import and export to be used at all customer sites. This
configuration specifies a simple topology wherein a customer has a single
routing domain which is shared across all its sites. More complex routing
topologies are possible through use of additional route-targets to augment the
leaking of sets of routes in various ways.

When using the shortcut syntax for vrf-to-vrf leaking, the RD and RT are
auto-derived.

General configuration
^^^^^^^^^^^^^^^^^^^^^

Configuration of route leaking between a unicast VRF RIB and the VPN SAFI RIB
of the default VRF is accomplished via commands in the context of a VRF
address-family:

.. clicmd:: rd vpn export AS:NN|IP:nn

   Specifies the route distinguisher to be added to a route exported from the
   current unicast VRF to VPN.

.. clicmd:: rt vpn import|export|both RTLIST...

   Specifies the route-target list to be attached to a route (export) or the
   route-target list to match against (import) when exporting/importing between
   the current unicast VRF and VPN.

   The RTLIST is a space-separated list of route-targets, which are BGP
   extended community values as described in
   :ref:`bgp-extended-communities-attribute`.

.. clicmd:: label vpn export (0..1048575)|auto

   Enables an MPLS label to be attached to a route exported from the current
   unicast VRF to VPN. If the value specified is ``auto``, the label value is
   automatically assigned from a pool maintained by the Zebra daemon. If Zebra
   is not running, or if this command is not configured, automatic label
   assignment will not complete, which will block corresponding route export.

.. clicmd:: nexthop vpn export A.B.C.D|X:X::X:X

   Specifies an optional nexthop value to be assigned to a route exported from
   the current unicast VRF to VPN. If left unspecified, the nexthop will be set
   to 0.0.0.0 or 0:0::0:0 (self).

.. clicmd:: route-map vpn import|export MAP

   Specifies an optional route-map to be applied to routes imported or exported
   between the current unicast VRF and VPN.

.. clicmd:: import|export vpn

   Enables import or export of routes between the current unicast VRF and VPN.

.. clicmd:: import vrf VRFNAME

   Shortcut syntax for specifying automatic leaking from vrf VRFNAME to
   the current VRF using the VPN RIB as intermediary.  The RD and RT
   are auto derived and should not be specified explicitly for either the
   source or destination VRF's.

   This shortcut syntax mode is not compatible with the explicit
   `import vpn` and `export vpn` statements for the two VRF's involved.
   The CLI will disallow attempts to configure incompatible leaking
   modes.

.. _bgp-l3vpn-srv6:

L3VPN SRv6
----------

.. clicmd:: segment-routing srv6

   Use SRv6 backend with BGP L3VPN, and go to its configuration node.

.. clicmd:: locator NAME

   Specify the SRv6 locator to be used for SRv6 L3VPN. The Locator name must
   be set in zebra, but user can set it in any order.

.. _bgp-evpn:

Ethernet Virtual Network - EVPN
-------------------------------

Note: When using EVPN features and if you have a large number of hosts, make
sure to adjust the size of the arp  neighbor cache to avoid neighbor table
overflow and/or excessive garbage collection. On Linux, the size of the table
and garbage collection frequency can be controlled via the following
sysctl configurations:

.. code-block:: shell

   net.ipv4.neigh.default.gc_thresh1
   net.ipv4.neigh.default.gc_thresh2
   net.ipv4.neigh.default.gc_thresh3

   net.ipv6.neigh.default.gc_thresh1
   net.ipv6.neigh.default.gc_thresh2
   net.ipv6.neigh.default.gc_thresh3

For more information, see ``man 7 arp``.

.. _bgp-evpn-advertise-pip:

EVPN advertise-PIP
^^^^^^^^^^^^^^^^^^

In a EVPN symmetric routing MLAG deployment, all EVPN routes advertised
with anycast-IP as next-hop IP and anycast MAC as the Router MAC (RMAC - in
BGP EVPN Extended-Community).
EVPN picks up the next-hop IP from the VxLAN interface's local tunnel IP and
the RMAC is obtained from the MAC of the L3VNI's SVI interface.
Note: Next-hop IP is used for EVPN routes whether symmetric routing is
deployed or not but the RMAC is only relevant for symmetric routing scenario.

Current behavior is not ideal for Prefix (type-5) and self (type-2)
routes. This is because the traffic from remote VTEPs routed sub optimally
if they land on the system where the route does not belong.

The advertise-pip feature advertises Prefix (type-5) and self (type-2)
routes with system's individual (primary) IP as the next-hop and individual
(system) MAC as Router-MAC (RMAC), while leaving the behavior unchanged for
other EVPN routes.

To support this feature there needs to have ability to co-exist a
(system-MAC, system-IP) pair with a (anycast-MAC, anycast-IP) pair with the
ability to terminate VxLAN-encapsulated packets received for either pair on
the same L3VNI (i.e associated VLAN). This capability is needed per tenant
VRF instance.

To derive the system-MAC and the anycast MAC, there must be a
separate/additional MAC-VLAN interface corresponding to L3VNI’s SVI.
The SVI interface’s MAC address can be interpreted as system-MAC
and MAC-VLAN interface's MAC as anycast MAC.

To derive system-IP and anycast-IP, the default BGP instance's router-id is used
as system-IP and the VxLAN interface’s local tunnel IP as the anycast-IP.

User has an option to configure the system-IP and/or system-MAC value if the
auto derived value is not preferred.

Note: By default, advertise-pip feature is enabled and user has an option to
disable the feature via configuration CLI. Once the feature is disabled under
bgp vrf instance or MAC-VLAN interface is not configured, all the routes follow
the same behavior of using same next-hop and RMAC values.

.. clicmd:: advertise-pip [ip <addr> [mac <addr>]]

Enables or disables advertise-pip feature, specifiy system-IP and/or system-MAC
parameters.

EVPN advertise-svi-ip
^^^^^^^^^^^^^^^^^^^^^
Typically, the SVI IP address is reused on VTEPs across multiple racks. However,
if you have unique SVI IP addresses that you want to be reachable you can use the
advertise-svi-ip option. This option advertises the SVI IP/MAC address as a type-2
route and eliminates the need for any flooding over VXLAN to reach the IP from a
remote VTEP.

.. clicmd:: advertise-svi-ip

Note that you should not enable both the advertise-svi-ip and the advertise-default-gw
at the same time.

.. _bgp-evpn-overlay-index-gateway-ip:

EVPN Overlay Index Gateway IP
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Draft https://tools.ietf.org/html/draft-ietf-bess-evpn-prefix-advertisement-11
explains the use of overlay indexes for recursive route resolution for EVPN
type-5 route.

We support gateway IP overlay index.
A gateway IP, advertised with EVPN prefix route, is used to find an EVPN MAC/IP
route with its IP field same as the gateway IP. This MAC/IP entry provides the
nexthop VTEP and the tunnel information required for the VxLAN encapsulation.

Functionality:

::

  .      +--------+ BGP  +--------+ BGP  +--------+      +--------+
    SN1  |        | IPv4 |        | EVPN |        |      |        |
   ======+ Host1  +------+   PE1  +------+   PE2  +------+  Host2 +
         |        |      |        |      |        |      |        |
         +--------+      +--------+      +--------+      +--------+

Consider above topology where prefix SN1 is connected behind host1. Host1
advertises SN1 to PE1 over BGP IPv4 session. PE1 advertises SN1 to PE2 using
EVPN type-5 route with host1 IP as the gateway IP. PE1 also advertises
Host1 MAC/IP as type-2 route which is used to resolve host1 gateway IP.

PE2 receives this type-5 route and imports it into the vrf based on route
targets. BGP prefix imported into the vrf uses gateway IP as its BGP nexthop.
This route is installed into zebra if following conditions are satisfied:

1. Gateway IP nexthop is L3 reachable.
2. PE2 has received EVPN type-2 route with IP field set to gateway IP.

Topology requirements:

1. This feature is supported for asymmetric routing model only. While
   sending packets to SN1, ingress PE (PE2) performs routing and
   egress PE (PE1) performs only bridging.
2. This feature supports only tratitional(non vlan-aware) bridge model. Bridge
   interface associated with L2VNI is an L3 interface. i.e., this interface is
   configured with an address in the L2VNI subnet. Note that the gateway IP
   should also have an address in the same subnet.
3. As this feature works in asymmetric routing model, all L2VNIs and corresponding
   VxLAN and bridge interfaces should be present at all the PEs.
4. L3VNI configuration is required to generate and import EVPN type-5 routes.
   L3VNI VxLAN and bridge interfaces also should be present.

A PE can use one of the following two mechanisms to advertise an EVPN type-5
route with gateway IP.

1. CLI to add gateway IP while generating EVPN type-5 route from a BGP IPv4/IPv6
prefix:

.. clicmd:: advertise <ipv4|ipv6> unicast [gateway-ip]

When this CLI is configured for a BGP vrf under L2VPN EVPN address family, EVPN
type-5 routes are generated for BGP prefixes in the vrf. Nexthop of the BGP
prefix becomes the gateway IP of the corresponding type-5 route.

If the above command is configured without the "gateway-ip" keyword, type-5
routes are generated without overlay index.

2. Add gateway IP to EVPN type-5 route using a route-map:

.. clicmd:: set evpn gateway-ip <ipv4|ipv6> <addr>

When route-map with above set clause is applied as outbound policy in BGP, it
will set the gateway-ip in EVPN type-5 NLRI.

Example configuration:

.. code-block:: frr

   router bgp 100
    neighbor 192.168.0.1 remote-as 101
    !
    address-family ipv4 l2vpn evpn
     neighbor 192.168.0.1 route-map RMAP out
    exit-address-family
   !
   route-map RMAP permit 10
    set evpn gateway-ip 10.0.0.1
    set evpn gateway-ip 10::1

A PE that receives a type-5 route with gateway IP overlay index should have
"enable-resolve-overlay-index" configuration enabled to recursively resolve the
overlay index nexthop and install the prefix into zebra.

.. clicmd:: enable-resolve-overlay-index

Example configuration:

.. code-block:: frr

   router bgp 65001
    bgp router-id 192.168.100.1
    no bgp ebgp-requires-policy
    neighbor 10.0.1.2 remote-as 65002
    !
    address-family l2vpn evpn
     neighbor 10.0.1.2 activate
     advertise-all-vni
     enable-resolve-overlay-index
    exit-address-family
   !

EVPN Multihoming
^^^^^^^^^^^^^^^^

All-Active Multihoming is used for redundancy and load sharing. Servers
are attached to two or more PEs and the links are bonded (link-aggregation).
This group of server links is referred to as an Ethernet Segment.

Ethernet Segments
"""""""""""""""""
An Ethernet Segment can be configured by specifying a system-MAC and a
local discriminatior against the bond interface on the PE (via zebra) -

.. clicmd:: evpn mh es-id (1-16777215)

.. clicmd:: evpn mh es-sys-mac X:X:X:X:X:X

The sys-mac and local discriminator are used for generating a 10-byte,
Type-3 Ethernet Segment ID.

Type-1 (EAS-per-ES and EAD-per-EVI) routes are used to advertise the locally
attached ESs and to learn off remote ESs in the network. Local Type-2/MAC-IP
routes are also advertised with a destination ESI allowing for MAC-IP syncing
between Ethernet Segment peers.
Reference: RFC 7432, RFC 8365

EVPN-MH is intended as a replacement for MLAG or Anycast VTEPs. In
multihoming each PE has an unique VTEP address which requires the introduction
of a new dataplane construct, MAC-ECMP. Here a MAC/FDB entry can point to a
list of remote PEs/VTEPs.

BUM handling
""""""""""""
Type-4 (ESR) routes are used for Designated Forwarder (DF) election. DFs
forward BUM traffic received via the overlay network. This implementation
uses a preference based DF election specified by draft-ietf-bess-evpn-pref-df.
The DF preference is configurable per-ES (via zebra) -

.. clicmd:: evpn mh es-df-pref (1-16777215)

BUM traffic is rxed via the overlay by all PEs attached to a server but
only the DF can forward the de-capsulated traffic to the access port. To
accomodate that non-DF filters are installed in the dataplane to drop
the traffic.

Similarly traffic received from ES peers via the overlay cannot be forwarded
to the server. This is split-horizon-filtering with local bias.

Knobs for interop
"""""""""""""""""
Some vendors do not send EAD-per-EVI routes. To interop with them we
need to relax the dependency on EAD-per-EVI routes and activate a remote
ES-PE based on just the EAD-per-ES route.

Note that by default we advertise and expect EAD-per-EVI routes.

.. clicmd:: disable-ead-evi-rx

.. clicmd:: disable-ead-evi-tx

Fast failover
"""""""""""""
As the primary purpose of EVPN-MH is redundancy keeping the failover efficient
is a recurring theme in the implementation. Following sub-features have
been introduced for the express purpose of efficient ES failovers.

- Layer-2 Nexthop Groups and MAC-ECMP via L2NHG.

- Host routes (for symmetric IRB) via L3NHG.
  On dataplanes that support layer3 nexthop groups the feature can be turned
  on via the following BGP config -

.. clicmd:: use-es-l3nhg

- Local ES (MAC/Neigh) failover via ES-redirect.
  On dataplanes that do not have support for ES-redirect the feature can be
  turned off via the following zebra config -

.. clicmd:: evpn mh redirect-off

Uplink/Core tracking
""""""""""""""""""""
When all the underlay links go down the PE no longer has access to the VxLAN
+overlay. To prevent blackholing of traffic the server/ES links are
protodowned on the PE. A link can be setup for uplink tracking via the
following zebra configuration -

.. clicmd:: evpn mh uplink

Proxy advertisements
""""""""""""""""""""
To handle hitless upgrades support for proxy advertisement has been added
as specified by draft-rbickhart-evpn-ip-mac-proxy-adv. This allows a PE
(say PE1) to proxy advertise a MAC-IP rxed from an ES peer (say PE2). When
the ES peer (PE2) goes down PE1 continues to advertise hosts learnt from PE2
for a holdtime during which it attempts to establish local reachability of
the host. This holdtime is configurable via the following zebra commands -

.. clicmd:: evpn mh neigh-holdtime (0-86400)

.. clicmd:: evpn mh mac-holdtime (0-86400)

Startup delay
"""""""""""""
When a switch is rebooted we wait for a brief period to allow the underlay
and EVPN network to converge before enabling the ESs. For this duration the
ES bonds are held protodown. The startup delay is configurable via the
following zebra command -

.. clicmd:: evpn mh startup-delay (0-3600)

+Support with VRF network namespace backend
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
It is possible to separate overlay networks contained in VXLAN interfaces from
underlay networks by using VRFs. VRF-lite and VRF-netns backends can be used for
that. In the latter case, it is necessary to set both bridge and vxlan interface
in the same network namespace, as below example illustrates:

.. code-block:: shell

   # linux shell
   ip netns add vrf1
   ip link add name vxlan101 type vxlan id 101 dstport 4789 dev eth0 local 10.1.1.1
   ip link set dev vxlan101 netns vrf1
   ip netns exec vrf1 ip link set dev lo up
   ip netns exec vrf1 brctl addbr bridge101
   ip netns exec vrf1 brctl addif bridge101 vxlan101

This makes it possible to separate not only layer 3 networks like VRF-lite networks.
Also, VRF netns based make possible to separate layer 2 networks on separate VRF
instances.

.. _bgp-conditional-advertisement:

BGP Conditional Advertisement
-----------------------------
The BGP conditional advertisement feature uses the ``non-exist-map`` or the
``exist-map`` and the ``advertise-map`` keywords of the neighbor advertise-map
command in order to track routes by the route prefix.

``non-exist-map``
   1. If a route prefix is not present in the output of non-exist-map command,
      then advertise the route specified by the advertise-map command.

   2. If a route prefix is present in the output of non-exist-map command,
      then do not advertise the route specified by the addvertise-map command.

``exist-map``
   1. If a route prefix is present in the output of exist-map command,
      then advertise the route specified by the advertise-map command.

   2. If a route prefix is not present in the output of exist-map command,
      then do not advertise the route specified by the advertise-map command.

This feature is useful when some prefixes are advertised to one of its peers
only if the information from the other peer is not present (due to failure in
peering session or partial reachability etc).

The conditional BGP announcements are sent in addition to the normal
announcements that a BGP router sends to its peer.

The conditional advertisement process is triggered by the BGP scanner process,
which runs every 60 by default. This means that the maximum time for the
conditional advertisement to take effect is the value of the process timer.

As an optimization, while the process always runs on each timer expiry, it
determines whether or not the conditional advertisement policy or the routing
table has changed; if neither have changed, no processing is necessary and the
scanner exits early.

.. clicmd:: neighbor A.B.C.D advertise-map NAME [exist-map|non-exist-map] NAME

   This command enables BGP scanner process to monitor routes specified by
   exist-map or non-exist-map command in BGP table and conditionally advertises
   the routes specified by advertise-map command.

.. clicmd:: bgp conditional-advertisement timer (5-240)

   Set the period to rerun the conditional advertisement scanner process. The
   default is 60 seconds.

Sample Configuration
^^^^^^^^^^^^^^^^^^^^^
.. code-block:: frr

   interface enp0s9
    ip address 10.10.10.2/24
   !
   interface enp0s10
    ip address 10.10.20.2/24
   !
   interface lo
    ip address 203.0.113.1/32
   !
   router bgp 2
    bgp log-neighbor-changes
    no bgp ebgp-requires-policy
    neighbor 10.10.10.1 remote-as 1
    neighbor 10.10.20.3 remote-as 3
    !
    address-family ipv4 unicast
     neighbor 10.10.10.1 soft-reconfiguration inbound
     neighbor 10.10.20.3 soft-reconfiguration inbound
     neighbor 10.10.20.3 advertise-map ADV-MAP non-exist-map EXIST-MAP
    exit-address-family
   !
   ip prefix-list DEFAULT seq 5 permit 192.0.2.5/32
   ip prefix-list DEFAULT seq 10 permit 192.0.2.1/32
   ip prefix-list EXIST seq 5 permit 10.10.10.10/32
   ip prefix-list DEFAULT-ROUTE seq 5 permit 0.0.0.0/0
   ip prefix-list IP1 seq 5 permit 10.139.224.0/20
   !
   bgp community-list standard DC-ROUTES seq 5 permit 64952:3008
   bgp community-list standard DC-ROUTES seq 10 permit 64671:501
   bgp community-list standard DC-ROUTES seq 15 permit 64950:3009
   bgp community-list standard DEFAULT-ROUTE seq 5 permit 65013:200
   !
   route-map ADV-MAP permit 10
    match ip address prefix-list IP1
   !
   route-map ADV-MAP permit 20
    match community DC-ROUTES
   !
   route-map EXIST-MAP permit 10
    match community DEFAULT-ROUTE
    match ip address prefix-list DEFAULT-ROUTE
   !

Sample Output
^^^^^^^^^^^^^

When default route is present in R2'2 BGP table, 10.139.224.0/20 and 192.0.2.1/32 are not advertised to R3.

.. code-block:: frr

   Router2# show ip bgp
   BGP table version is 20, local router ID is 203.0.113.1, vrf id 0
   Default local pref 100, local AS 2
   Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
                  i internal, r RIB-failure, S Stale, R Removed
   Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
   Origin codes:  i - IGP, e - EGP, ? - incomplete
   RPKI validation codes: V valid, I invalid, N Not found

      Network          Next Hop            Metric LocPrf Weight Path
   *> 0.0.0.0/0        10.10.10.1               0             0 1 i
   *> 10.139.224.0/20  10.10.10.1               0             0 1 ?
   *> 192.0.2.1/32     10.10.10.1               0             0 1 i
   *> 192.0.2.5/32     10.10.10.1               0             0 1 i

   Displayed  4 routes and 4 total paths
   Router2# show ip bgp neighbors 10.10.20.3

   !--- Output suppressed.

   For address family: IPv4 Unicast
   Update group 7, subgroup 7
   Packet Queue length 0
   Inbound soft reconfiguration allowed
   Community attribute sent to this neighbor(all)
   Condition NON_EXIST, Condition-map *EXIST-MAP, Advertise-map *ADV-MAP, status: Withdraw
   0 accepted prefixes

   !--- Output suppressed.

   Router2# show ip bgp neighbors 10.10.20.3 advertised-routes
   BGP table version is 20, local router ID is 203.0.113.1, vrf id 0
   Default local pref 100, local AS 2
   Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
   Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
   Origin codes:  i - IGP, e - EGP, ? - incomplete
   RPKI validation codes: V valid, I invalid, N Not found

      Network          Next Hop            Metric LocPrf Weight Path
   *> 0.0.0.0/0        0.0.0.0                                0 1 i
   *> 192.0.2.5/32     0.0.0.0                                0 1 i

   Total number of prefixes 2

When default route is not present in R2'2 BGP table, 10.139.224.0/20 and 192.0.2.1/32 are advertised to R3.

.. code-block:: frr

   Router2# show ip bgp
   BGP table version is 21, local router ID is 203.0.113.1, vrf id 0
   Default local pref 100, local AS 2
   Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
                  i internal, r RIB-failure, S Stale, R Removed
   Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
   Origin codes:  i - IGP, e - EGP, ? - incomplete
   RPKI validation codes: V valid, I invalid, N Not found

      Network          Next Hop            Metric LocPrf Weight Path
   *> 10.139.224.0/20  10.10.10.1               0             0 1 ?
   *> 192.0.2.1/32     10.10.10.1               0             0 1 i
   *> 192.0.2.5/32     10.10.10.1               0             0 1 i

   Displayed  3 routes and 3 total paths

   Router2# show ip bgp neighbors 10.10.20.3

   !--- Output suppressed.

   For address family: IPv4 Unicast
   Update group 7, subgroup 7
   Packet Queue length 0
   Inbound soft reconfiguration allowed
   Community attribute sent to this neighbor(all)
   Condition NON_EXIST, Condition-map *EXIST-MAP, Advertise-map *ADV-MAP, status: Advertise
   0 accepted prefixes

   !--- Output suppressed.

   Router2# show ip bgp neighbors 10.10.20.3 advertised-routes
   BGP table version is 21, local router ID is 203.0.113.1, vrf id 0
   Default local pref 100, local AS 2
   Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
                  i internal, r RIB-failure, S Stale, R Removed
   Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
   Origin codes:  i - IGP, e - EGP, ? - incomplete
   RPKI validation codes: V valid, I invalid, N Not found

      Network          Next Hop            Metric LocPrf Weight Path
   *> 10.139.224.0/20  0.0.0.0                                0 1 ?
   *> 192.0.2.1/32     0.0.0.0                                0 1 i
   *> 192.0.2.5/32     0.0.0.0                                0 1 i

   Total number of prefixes 3
   Router2#

.. _bgp-debugging:

Debugging
---------

.. clicmd:: show debug

   Show all enabled debugs.

.. clicmd:: show bgp listeners

   Display Listen sockets and the vrf that created them.  Useful for debugging of when
   listen is not working and this is considered a developer debug statement.

.. clicmd:: debug bgp bfd

   Enable or disable debugging for BFD events. This will show BFD integration
   library messages and BGP BFD integration messages that are mostly state
   transitions and validation problems.

.. clicmd:: debug bgp neighbor-events

   Enable or disable debugging for neighbor events. This provides general
   information on BGP events such as peer connection / disconnection, session
   establishment / teardown, and capability negotiation.

.. clicmd:: debug bgp updates

   Enable or disable debugging for BGP updates. This provides information on
   BGP UPDATE messages transmitted and received between local and remote
   instances.

.. clicmd:: debug bgp keepalives

   Enable or disable debugging for BGP keepalives. This provides information on
   BGP KEEPALIVE messages transmitted and received between local and remote
   instances.

.. clicmd:: debug bgp bestpath <A.B.C.D/M|X:X::X:X/M>

   Enable or disable debugging for bestpath selection on the specified prefix.

.. clicmd:: debug bgp nht

   Enable or disable debugging of BGP nexthop tracking.

.. clicmd:: debug bgp update-groups

   Enable or disable debugging of dynamic update groups. This provides general
   information on group creation, deletion, join and prune events.

.. clicmd:: debug bgp zebra

   Enable or disable debugging of communications between *bgpd* and *zebra*.

Dumping Messages and Routing Tables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. clicmd:: dump bgp all PATH [INTERVAL]

.. clicmd:: dump bgp all-et PATH [INTERVAL]


   Dump all BGP packet and events to `path` file.
   If `interval` is set, a new file will be created for echo `interval` of
   seconds.  The path `path` can be set with date and time formatting
   (strftime).  The type ‘all-et’ enables support for Extended Timestamp Header
   (:ref:`packet-binary-dump-format`).

.. clicmd:: dump bgp updates PATH [INTERVAL]

.. clicmd:: dump bgp updates-et PATH [INTERVAL]


   Dump only BGP updates messages to `path` file.
   If `interval` is set, a new file will be created for echo `interval` of
   seconds.  The path `path` can be set with date and time formatting
   (strftime).  The type ‘updates-et’ enables support for Extended Timestamp
   Header (:ref:`packet-binary-dump-format`).

.. clicmd:: dump bgp routes-mrt PATH

.. clicmd:: dump bgp routes-mrt PATH INTERVAL


   Dump whole BGP routing table to `path`. This is heavy process. The path
   `path` can be set with date and time formatting (strftime). If `interval` is
   set, a new file will be created for echo `interval` of seconds.

   Note: the interval variable can also be set using hours and minutes: 04h20m00.


.. _bgp-other-commands:

Other BGP Commands
------------------

The following are available in the top level *enable* mode:

.. clicmd:: clear bgp \*

   Clear all peers.

.. clicmd:: clear bgp ipv4|ipv6 \*

   Clear all peers with this address-family activated.

.. clicmd:: clear bgp ipv4|ipv6 unicast \*

   Clear all peers with this address-family and sub-address-family activated.

.. clicmd:: clear bgp ipv4|ipv6 PEER

   Clear peers with address of X.X.X.X and this address-family activated.

.. clicmd:: clear bgp ipv4|ipv6 unicast PEER

   Clear peer with address of X.X.X.X and this address-family and sub-address-family activated.

.. clicmd:: clear bgp ipv4|ipv6 PEER soft|in|out

   Clear peer using soft reconfiguration in this address-family.

.. clicmd:: clear bgp ipv4|ipv6 unicast PEER soft|in|out

   Clear peer using soft reconfiguration in this address-family and sub-address-family.

The following are available in the ``router bgp`` mode:

.. clicmd:: write-quanta (1-64)

   BGP message Tx I/O is vectored. This means that multiple packets are written
   to the peer socket at the same time each I/O cycle, in order to minimize
   system call overhead. This value controls how many are written at a time.
   Under certain load conditions, reducing this value could make peer traffic
   less 'bursty'. In practice, leave this settings on the default (64) unless
   you truly know what you are doing.

.. clicmd:: read-quanta (1-10)

   Unlike Tx, BGP Rx traffic is not vectored. Packets are read off the wire one
   at a time in a loop. This setting controls how many iterations the loop runs
   for. As with write-quanta, it is best to leave this setting on the default.

The following command is available in ``config`` mode as well as in the
``router bgp`` mode:

.. clicmd:: bgp graceful-shutdown

   The purpose of this command is to initiate BGP Graceful Shutdown which
   is described in :rfc:`8326`. The use case for this is to minimize or
   eliminate the amount of traffic loss in a network when a planned
   maintenance activity such as software upgrade or hardware replacement
   is to be performed on a router. The feature works by re-announcing
   routes to eBGP peers with the GRACEFUL_SHUTDOWN community included.
   Peers are then expected to treat such paths with the lowest preference.
   This happens automatically on a receiver running FRR; with other
   routing protocol stacks, an inbound policy may have to be configured.
   In FRR, triggering graceful shutdown also results in announcing a
   LOCAL_PREF of 0 to iBGP peers.

   Graceful shutdown can be configured per BGP instance or globally for
   all of BGP. These two options are mutually exclusive. The no form of
   the command causes graceful shutdown to be stopped, and routes will
   be re-announced without the GRACEFUL_SHUTDOWN community and/or with
   the usual LOCAL_PREF value. Note that if this option is saved to
   the startup configuration, graceful shutdown will remain in effect
   across restarts of *bgpd* and will need to be explicitly disabled.

.. _bgp-displaying-bgp-information:

Displaying BGP Information
==========================

The following four commands display the IPv6 and IPv4 routing tables, depending
on whether or not the ``ip`` keyword is used.
Actually, :clicmd:`show ip bgp` command was used on older `Quagga` routing
daemon project, while :clicmd:`show bgp` command is the new format. The choice
has been done to keep old format with IPv4 routing table, while new format
displays IPv6 routing table.

.. clicmd:: show ip bgp [all] [wide|json [detail]]

.. clicmd:: show ip bgp A.B.C.D [json]

.. clicmd:: show bgp [all] [wide|json [detail]]

.. clicmd:: show bgp X:X::X:X [json]

   These commands display BGP routes. When no route is specified, the default
   is to display all BGP routes.

   ::

      BGP table version is 0, local router ID is 10.1.1.1
         Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
         Origin codes: i - IGP, e - EGP, ? - incomplete

      Network    Next Hop      Metric LocPrf Weight Path
         \*> 1.1.1.1/32       0.0.0.0      0   32768 i

         Total number of prefixes 1

   If ``wide`` option is specified, then the prefix table's width is increased
   to fully display the prefix and the nexthop.

   This is especially handy dealing with IPv6 prefixes and
   if :clicmd:`[no] bgp default show-nexthop-hostname` is enabled.

   If ``all`` option is specified, ``ip`` keyword is ignored, show bgp all and
   show ip bgp all commands display routes for all AFIs and SAFIs.

   If ``json`` option is specified, output is displayed in JSON format.

   If ``detail`` option is specified after ``json``, more verbose JSON output
   will be displayed.

Some other commands provide additional options for filtering the output.

.. clicmd:: show [ip] bgp regexp LINE

   This command displays BGP routes using AS path regular expression
   (:ref:`bgp-regular-expressions`).

.. clicmd:: show [ip] bgp [all] summary [wide] [json]

   Show a bgp peer summary for the specified address family.

The old command structure :clicmd:`show ip bgp` may be removed in the future
and should no longer be used. In order to reach the other BGP routing tables
other than the IPv6 routing table given by :clicmd:`show bgp`, the new command
structure is extended with :clicmd:`show bgp [afi] [safi]`.

``wide`` option gives more output like ``LocalAS`` and extended ``Desc`` to
64 characters.

   .. code-block:: frr

      exit1# show ip bgp summary wide

      IPv4 Unicast Summary (VRF default):
      BGP router identifier 192.168.100.1, local AS number 65534 vrf-id 0
      BGP table version 3
      RIB entries 5, using 920 bytes of memory
      Peers 1, using 27 KiB of memory

      Neighbor        V         AS    LocalAS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt Desc
      192.168.0.2     4      65030        123        15        22        0    0    0 00:07:00            0        1 us-east1-rs1.frrouting.org

      Total number of neighbors 1
      exit1#

.. clicmd:: show bgp [afi] [safi] [all] [wide|json]

.. clicmd:: show bgp [<ipv4|ipv6> <unicast|multicast|vpn|labeled-unicast|flowspec> | l2vpn evpn]

   These commands display BGP routes for the specific routing table indicated by
   the selected afi and the selected safi. If no afi and no safi value is given,
   the command falls back to the default IPv6 routing table.

.. clicmd:: show bgp l2vpn evpn route [type <macip|2|multicast|3|es|4|prefix|5>]

   EVPN prefixes can also be filtered by EVPN route type.

.. clicmd:: show bgp [afi] [safi] [all] summary [json]

   Show a bgp peer summary for the specified address family, and subsequent
   address-family.

.. clicmd:: show bgp [afi] [safi] [all] summary failed [json]

   Show a bgp peer summary for peers that are not succesfully exchanging routes
   for the specified address family, and subsequent address-family.

.. clicmd:: show bgp [afi] [safi] [all] summary established [json]

   Show a bgp peer summary for peers that are succesfully exchanging routes
   for the specified address family, and subsequent address-family.

.. clicmd:: show bgp [afi] [safi] [all] summary neighbor [PEER] [json]

   Show a bgp summary for the specified peer, address family, and
   subsequent address-family. The neighbor filter can be used in combination
   with the failed, established filters.

.. clicmd:: show bgp [afi] [safi] [all] summary remote-as <internal|external|ASN> [json]

   Show a bgp peer summary for the specified remote-as ASN or type (``internal``
   for iBGP and ``external`` for eBGP sessions), address family, and subsequent
   address-family. The remote-as filter can be used in combination with the
   failed, established filters.

.. clicmd:: show bgp [afi] [safi] [all] summary terse [json]

   Shorten the output. Do not show the following information about the BGP
   instances: the number of RIB entries, the table version and the used memory.
   The ``terse`` option can be used in combination with the remote-as, neighbor,
   failed and established filters, and with the ``wide`` option as well.

.. clicmd:: show bgp [afi] [safi] [neighbor [PEER] [routes|advertised-routes|received-routes] [json]

   This command shows information on a specific BGP peer of the relevant
   afi and safi selected.

   The ``routes`` keyword displays only routes in this address-family's BGP
   table that were received by this peer and accepted by inbound policy.

   The ``advertised-routes`` keyword displays only the routes in this
   address-family's BGP table that were permitted by outbound policy and
   advertised to to this peer.

   The ``received-routes`` keyword displays all routes belonging to this
   address-family (prior to inbound policy) that were received by this peer.

.. clicmd:: show bgp [afi] [safi] [all] dampening dampened-paths [wide|json]

   Display paths suppressed due to dampening of the selected afi and safi
   selected.

.. clicmd:: show bgp [afi] [safi] [all] dampening flap-statistics [wide|json]

   Display flap statistics of routes of the selected afi and safi selected.

.. clicmd:: show bgp [afi] [safi] [all] version (1-4294967295) [wide|json]

   Display prefixes with matching version numbers. The version number and
   above having prefixes will be listed here.

   It helps to identify which prefixes were installed at some point.

   Here is an example of how to check what prefixes were installed starting
   with an arbitrary version:

.. code-block:: shell

   # vtysh -c 'show bgp ipv4 unicast json' | jq '.tableVersion'
   9
   # vtysh -c 'show ip bgp version 9 json' | jq -r '.routes | keys[]'
   192.168.3.0/24
   # vtysh -c 'show ip bgp version 8 json' | jq -r '.routes | keys[]'
   192.168.2.0/24
   192.168.3.0/24

.. clicmd:: show bgp [afi] [safi] statistics

   Display statistics of routes of the selected afi and safi.

.. clicmd:: show bgp statistics-all

   Display statistics of routes of all the afi and safi.

.. clicmd:: show [ip] bgp [afi] [safi] [all] cidr-only [wide|json]

   Display routes with non-natural netmasks.

.. clicmd:: show [ip] bgp [afi] [safi] [all] neighbors A.B.C.D [advertised-routes|received-routes|filtered-routes] [json|wide]

   Display the routes advertised to a BGP neighbor or received routes
   from neighbor or filtered routes received from neighbor based on the
   option specified.

   If ``wide`` option is specified, then the prefix table's width is increased
   to fully display the prefix and the nexthop.

   This is especially handy dealing with IPv6 prefixes and
   if :clicmd:`[no] bgp default show-nexthop-hostname` is enabled.

   If ``all`` option is specified, ``ip`` keyword is ignored and,
   routes displayed for all AFIs and SAFIs.
   if afi is specified, with ``all`` option, routes will be displayed for
   each SAFI in the selcted AFI

   If ``json`` option is specified, output is displayed in JSON format.

.. _bgp-display-routes-by-community:

Displaying Routes by Community Attribute
----------------------------------------

The following commands allow displaying routes based on their community
attribute.

.. clicmd:: show [ip] bgp <ipv4|ipv6> [all] community [wide|json]

.. clicmd:: show [ip] bgp <ipv4|ipv6> [all] community COMMUNITY [wide|json]

.. clicmd:: show [ip] bgp <ipv4|ipv6> [all] community COMMUNITY exact-match [wide|json]

   These commands display BGP routes which have the community attribute.
   attribute. When ``COMMUNITY`` is specified, BGP routes that match that
   community are displayed. When `exact-match` is specified, it display only
   routes that have an exact match.

.. clicmd:: show [ip] bgp <ipv4|ipv6> community-list WORD

.. clicmd:: show [ip] bgp <ipv4|ipv6> community-list WORD exact-match

   These commands display BGP routes for the address family specified that
   match the specified community list. When `exact-match` is specified, it
   displays only routes that have an exact match.

   If ``wide`` option is specified, then the prefix table's width is increased
   to fully display the prefix and the nexthop.

   This is especially handy dealing with IPv6 prefixes and
   if :clicmd:`[no] bgp default show-nexthop-hostname` is enabled.

   If ``all`` option is specified, ``ip`` keyword is ignored and,
   routes displayed for all AFIs and SAFIs.
   if afi is specified, with ``all`` option, routes will be displayed for
   each SAFI in the selcted AFI

   If ``json`` option is specified, output is displayed in JSON format.

.. clicmd:: show bgp labelpool <chunks|inuse|ledger|requests|summary> [json]

   These commands display information about the BGP labelpool used for
   the association of MPLS labels with routes for L3VPN and Labeled Unicast

   If ``chunks`` option is specified, output shows the current list of label
   chunks granted to BGP by Zebra, indicating the start and end label in
   each chunk

   If ``inuse`` option is specified, output shows the current inuse list of
   label to prefix mappings

   If ``ledger`` option is specified, output shows ledger list of all
   label requests made per prefix

   If ``requests`` option is specified, output shows current list of label
   requests which have not yet been fulfilled by the labelpool

   If ``summary`` option is specified, output is a summary of the counts for
   the chunks, inuse, ledger and requests list along with the count of
   outstanding chunk requests to Zebra and the nummber of zebra reconnects
   that have happened

   If ``json`` option is specified, output is displayed in JSON format.

.. _bgp-display-routes-by-lcommunity:

Displaying Routes by Large Community Attribute
----------------------------------------------

The following commands allow displaying routes based on their
large community attribute.

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community LARGE-COMMUNITY

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community LARGE-COMMUNITY exact-match

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community LARGE-COMMUNITY json

   These commands display BGP routes which have the large community attribute.
   attribute. When ``LARGE-COMMUNITY`` is specified, BGP routes that match that
   large community are displayed. When `exact-match` is specified, it display
   only routes that have an exact match. When `json` is specified, it display
   routes in json format.

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community-list WORD

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community-list WORD exact-match

.. clicmd:: show [ip] bgp <ipv4|ipv6> large-community-list WORD json

   These commands display BGP routes for the address family specified that
   match the specified large community list. When `exact-match` is specified,
   it displays only routes that have an exact match. When `json` is specified,
   it display routes in json format.

.. _bgp-display-routes-by-as-path:


Displaying Routes by AS Path
----------------------------

.. clicmd:: show bgp ipv4|ipv6 regexp LINE

   This commands displays BGP routes that matches a regular
   expression `line` (:ref:`bgp-regular-expressions`).

.. clicmd:: show [ip] bgp ipv4 vpn

.. clicmd:: show [ip] bgp ipv6 vpn

   Print active IPV4 or IPV6 routes advertised via the VPN SAFI.

.. clicmd:: show bgp ipv4 vpn summary

.. clicmd:: show bgp ipv6 vpn summary

   Print a summary of neighbor connections for the specified AFI/SAFI combination.

Displaying Routes by Route Distinguisher
----------------------------------------

.. clicmd:: show bgp [<ipv4|ipv6> vpn | l2vpn evpn [route]] rd <all|RD>

   For L3VPN and EVPN address-families, routes can be displayed on a per-RD
   (Route Distinguisher) basis or for all RD's.

.. clicmd:: show bgp l2vpn evpn rd <all|RD> [overlay | tags]

   Use the ``overlay`` or ``tags`` keywords to display the overlay/tag
   information about the EVPN prefixes in the selected Route Distinguisher.

.. clicmd:: show bgp l2vpn evpn route rd <all|RD> mac <MAC> [ip <MAC>] [json]

   For EVPN Type 2 (macip) routes, a MAC address (and optionally an IP address)
   can be supplied to the command to only display matching prefixes in the
   specified RD.

Displaying Update Group Information
-----------------------------------

.. clicmd:: show bgp update-groups [advertise-queue|advertised-routes|packet-queue]

   Display Information about each individual update-group being used.
   If SUBGROUP-ID is specified only display about that particular group.  If
   advertise-queue is specified the list of routes that need to be sent
   to the peers in the update-group is displayed, advertised-routes means
   the list of routes we have sent to the peers in the update-group and
   packet-queue specifies the list of packets in the queue to be sent.

.. clicmd:: show bgp update-groups statistics

   Display Information about update-group events in FRR.

Segment-Routing IPv6
--------------------

.. clicmd:: show bgp segment-routing srv6

   This command displays information about SRv6 L3VPN in bgpd.  Specifically,
   what kind of Locator is being used, and its Locator chunk information.
   And the SID of the SRv6 Function that is actually managed on bgpd.
   In the following example, bgpd is using a Locator named loc1, and two SRv6
   Functions are managed to perform VPNv6 VRF redirect for vrf10 and vrf20.

::

   router# show bgp segment-routing srv6
   locator_name: loc1
   locator_chunks:
   - 2001:db8:1:1::/64
   functions:
   - sid: 2001:db8:1:1::100
     locator: loc1
   - sid: 2001:db8:1:1::200
     locator: loc1
   bgps:
   - name: default
     vpn_policy[AFI_IP].tovpn_sid: none
     vpn_policy[AFI_IP6].tovpn_sid: none
   - name: vrf10
     vpn_policy[AFI_IP].tovpn_sid: none
     vpn_policy[AFI_IP6].tovpn_sid: 2001:db8:1:1::100
   - name: vrf20
     vpn_policy[AFI_IP].tovpn_sid: none
     vpn_policy[AFI_IP6].tovpn_sid: 2001:db8:1:1::200


.. _bgp-route-reflector:

Route Reflector
===============

BGP routers connected inside the same AS through BGP belong to an internal
BGP session, or IBGP. In order to prevent routing table loops, IBGP does not
advertise IBGP-learned routes to other routers in the same session. As such,
IBGP requires a full mesh of all peers. For large networks, this quickly becomes
unscalable. Introducing route reflectors removes the need for the full-mesh.

When route reflectors are configured, these will reflect the routes announced
by the peers configured as clients. A route reflector client is configured
with:

.. clicmd:: neighbor PEER route-reflector-client


To avoid single points of failure, multiple route reflectors can be configured.

A cluster is a collection of route reflectors and their clients, and is used
by route reflectors to avoid looping.

.. clicmd:: bgp cluster-id A.B.C.D

.. clicmd:: bgp no-rib

To set and unset the BGP daemon ``-n`` / ``--no_kernel`` options during runtime
to disable BGP route installation to the RIB (Zebra), the ``[no] bgp no-rib``
commands can be used;

Please note that setting the option during runtime will withdraw all routes in
the daemons RIB from Zebra and unsetting it will announce all routes in the
daemons RIB to Zebra. If the option is passed as a command line argument when
starting the daemon and the configuration gets saved, the option will persist
unless removed from the configuration with the negating command prior to the
configuration write operation.

.. clicmd:: bgp send-extra-data zebra

This Command turns off the ability of BGP to send extra data to zebra.
In this case it's the AS-Path being used for the path.  The default behavior
in BGP is to send this data and to turn it off enter the no form of the command.
If extra data was sent to zebra, and this command is turned on there is no
effort to clean up this data in the rib.

.. _bgp-suppress-fib:

Suppressing routes not installed in FIB
=======================================

The FRR implementation of BGP advertises prefixes learnt from a peer to other
peers even if the routes do not get installed in the FIB. There can be
scenarios where the hardware tables in some of the routers (along the path from
the source to destination) is full which will result in all routes not getting
installed in the FIB. If these routes are advertised to the downstream routers
then traffic will start flowing and will be dropped at the intermediate router.

The solution is to provide a configurable option to check for the FIB install
status of the prefixes and advertise to peers if the prefixes are successfully
installed in the FIB. The advertisement of the prefixes are suppressed if it is
not installed in FIB.

The following conditions apply will apply when checking for route installation
status in FIB:

1. The advertisement or suppression of routes based on FIB install status
   applies only for newly learnt routes from peer (routes which are not in
   BGP local RIB).
2. If the route received from peer already exists in BGP local RIB and route
   attributes have changed (best path changed), the old path is deleted and
   new path is installed in FIB. The FIB install status will not have any
   effect. Therefore only when the route is received first time the checks
   apply.
3. The feature will not apply for routes learnt through other means like
   redistribution to bgp from other protocols. This is applicable only to
   peer learnt routes.
4. If a route is installed in FIB and then gets deleted from the dataplane,
   then routes will not be withdrawn from peers. This will be considered as
   dataplane issue.
5. The feature will slightly increase the time required to advertise the routes
   to peers since the route install status needs to be received from the FIB
6. If routes are received by the peer before the configuration is applied, then
   the bgp sessions need to be reset for the configuration to take effect.
7. If the route which is already installed in dataplane is removed for some
   reason, sending withdraw message to peers is not currently supported.

.. clicmd:: bgp suppress-fib-pending

   This command is applicable at the global level and at an individual
   bgp level.  If applied at the global level all bgp instances will
   wait for fib installation before announcing routes and there is no
   way to turn it off for a particular bgp vrf.

.. _routing-policy:

Routing Policy
==============

You can set different routing policy for a peer. For example, you can set
different filter for a peer.

.. code-block:: frr

   !
   router bgp 1 view 1
    neighbor 10.0.0.1 remote-as 2
    address-family ipv4 unicast
     neighbor 10.0.0.1 distribute-list 1 in
    exit-address-family
   !
   router bgp 1 view 2
    neighbor 10.0.0.1 remote-as 2
    address-family ipv4 unicast
     neighbor 10.0.0.1 distribute-list 2 in
    exit-address-family

This means BGP update from a peer 10.0.0.1 goes to both BGP view 1 and view 2.
When the update is inserted into view 1, distribute-list 1 is applied. On the
other hand, when the update is inserted into view 2, distribute-list 2 is
applied.


.. _bgp-regular-expressions:

BGP Regular Expressions
=======================

BGP regular expressions are based on :t:`POSIX 1003.2` regular expressions. The
following description is just a quick subset of the POSIX regular expressions.


.\*
   Matches any single character.

\*
   Matches 0 or more occurrences of pattern.

\+
   Matches 1 or more occurrences of pattern.

?
   Match 0 or 1 occurrences of pattern.

^
   Matches the beginning of the line.

$
   Matches the end of the line.

_
   The ``_`` character has special meanings in BGP regular expressions.  It
   matches to space and comma , and AS set delimiter ``{`` and ``}`` and AS
   confederation delimiter ``(`` and ``)``. And it also matches to the
   beginning of the line and the end of the line. So ``_`` can be used for AS
   value boundaries match. This character technically evaluates to
   ``(^|[,{}()]|$)``.


.. _bgp-configuration-examples:

Miscellaneous Configuration Examples
====================================

Example of a session to an upstream, advertising only one prefix to it.

.. code-block:: frr

   router bgp 64512
    bgp router-id 10.236.87.1
    neighbor upstream peer-group
    neighbor upstream remote-as 64515
    neighbor upstream capability dynamic
    neighbor 10.1.1.1 peer-group upstream
    neighbor 10.1.1.1 description ACME ISP

    address-family ipv4 unicast
     network 10.236.87.0/24
     neighbor upstream prefix-list pl-allowed-adv out
    exit-address-family
   !
   ip prefix-list pl-allowed-adv seq 5 permit 82.195.133.0/25
   ip prefix-list pl-allowed-adv seq 10 deny any

A more complex example including upstream, peer and customer sessions
advertising global prefixes and NO_EXPORT prefixes and providing actions for
customer routes based on community values. Extensive use is made of route-maps
and the 'call' feature to support selective advertising of prefixes. This
example is intended as guidance only, it has NOT been tested and almost
certainly contains silly mistakes, if not serious flaws.

.. code-block:: frr

   router bgp 64512
    bgp router-id 10.236.87.1
    neighbor upstream capability dynamic
    neighbor cust capability dynamic
    neighbor peer capability dynamic
    neighbor 10.1.1.1 remote-as 64515
    neighbor 10.1.1.1 peer-group upstream
    neighbor 10.2.1.1 remote-as 64516
    neighbor 10.2.1.1 peer-group upstream
    neighbor 10.3.1.1 remote-as 64517
    neighbor 10.3.1.1 peer-group cust-default
    neighbor 10.3.1.1 description customer1
    neighbor 10.4.1.1 remote-as 64518
    neighbor 10.4.1.1 peer-group cust
    neighbor 10.4.1.1 description customer2
    neighbor 10.5.1.1 remote-as 64519
    neighbor 10.5.1.1 peer-group peer
    neighbor 10.5.1.1 description peer AS 1
    neighbor 10.6.1.1 remote-as 64520
    neighbor 10.6.1.1 peer-group peer
    neighbor 10.6.1.1 description peer AS 2

    address-family ipv4 unicast
     network 10.123.456.0/24
     network 10.123.456.128/25 route-map rm-no-export
     neighbor upstream route-map rm-upstream-out out
     neighbor cust route-map rm-cust-in in
     neighbor cust route-map rm-cust-out out
     neighbor cust send-community both
     neighbor peer route-map rm-peer-in in
     neighbor peer route-map rm-peer-out out
     neighbor peer send-community both
     neighbor 10.3.1.1 prefix-list pl-cust1-network in
     neighbor 10.4.1.1 prefix-list pl-cust2-network in
     neighbor 10.5.1.1 prefix-list pl-peer1-network in
     neighbor 10.6.1.1 prefix-list pl-peer2-network in
    exit-address-family
   !
   ip prefix-list pl-default permit 0.0.0.0/0
   !
   ip prefix-list pl-upstream-peers permit 10.1.1.1/32
   ip prefix-list pl-upstream-peers permit 10.2.1.1/32
   !
   ip prefix-list pl-cust1-network permit 10.3.1.0/24
   ip prefix-list pl-cust1-network permit 10.3.2.0/24
   !
   ip prefix-list pl-cust2-network permit 10.4.1.0/24
   !
   ip prefix-list pl-peer1-network permit 10.5.1.0/24
   ip prefix-list pl-peer1-network permit 10.5.2.0/24
   ip prefix-list pl-peer1-network permit 192.168.0.0/24
   !
   ip prefix-list pl-peer2-network permit 10.6.1.0/24
   ip prefix-list pl-peer2-network permit 10.6.2.0/24
   ip prefix-list pl-peer2-network permit 192.168.1.0/24
   ip prefix-list pl-peer2-network permit 192.168.2.0/24
   ip prefix-list pl-peer2-network permit 172.16.1/24
   !
   bgp as-path access-list seq 5 asp-own-as permit ^$
   bgp as-path access-list seq 10 asp-own-as permit _64512_
   !
   ! #################################################################
   ! Match communities we provide actions for, on routes receives from
   ! customers. Communities values of <our-ASN>:X, with X, have actions:
   !
   ! 100 - blackhole the prefix
   ! 200 - set no_export
   ! 300 - advertise only to other customers
   ! 400 - advertise only to upstreams
   ! 500 - set no_export when advertising to upstreams
   ! 2X00 - set local_preference to X00
   !
   ! blackhole the prefix of the route
   bgp community-list standard cm-blackhole permit 64512:100
   !
   ! set no-export community before advertising
   bgp community-list standard cm-set-no-export permit 64512:200
   !
   ! advertise only to other customers
   bgp community-list standard cm-cust-only permit 64512:300
   !
   ! advertise only to upstreams
   bgp community-list standard cm-upstream-only permit 64512:400
   !
   ! advertise to upstreams with no-export
   bgp community-list standard cm-upstream-noexport permit 64512:500
   !
   ! set local-pref to least significant 3 digits of the community
   bgp community-list standard cm-prefmod-100 permit 64512:2100
   bgp community-list standard cm-prefmod-200 permit 64512:2200
   bgp community-list standard cm-prefmod-300 permit 64512:2300
   bgp community-list standard cm-prefmod-400 permit 64512:2400
   bgp community-list expanded cme-prefmod-range permit 64512:2...
   !
   ! Informational communities
   !
   ! 3000 - learned from upstream
   ! 3100 - learned from customer
   ! 3200 - learned from peer
   !
   bgp community-list standard cm-learnt-upstream permit 64512:3000
   bgp community-list standard cm-learnt-cust permit 64512:3100
   bgp community-list standard cm-learnt-peer permit 64512:3200
   !
   ! ###################################################################
   ! Utility route-maps
   !
   ! These utility route-maps generally should not used to permit/deny
   ! routes, i.e. they do not have meaning as filters, and hence probably
   ! should be used with 'on-match next'. These all finish with an empty
   ! permit entry so as not interfere with processing in the caller.
   !
   route-map rm-no-export permit 10
    set community additive no-export
   route-map rm-no-export permit 20
   !
   route-map rm-blackhole permit 10
    description blackhole, up-pref and ensure it cannot escape this AS
    set ip next-hop 127.0.0.1
    set local-preference 10
    set community additive no-export
   route-map rm-blackhole permit 20
   !
   ! Set local-pref as requested
   route-map rm-prefmod permit 10
    match community cm-prefmod-100
    set local-preference 100
   route-map rm-prefmod permit 20
    match community cm-prefmod-200
    set local-preference 200
   route-map rm-prefmod permit 30
    match community cm-prefmod-300
    set local-preference 300
   route-map rm-prefmod permit 40
    match community cm-prefmod-400
    set local-preference 400
   route-map rm-prefmod permit 50
   !
   ! Community actions to take on receipt of route.
   route-map rm-community-in permit 10
    description check for blackholing, no point continuing if it matches.
    match community cm-blackhole
    call rm-blackhole
   route-map rm-community-in permit 20
    match community cm-set-no-export
    call rm-no-export
    on-match next
   route-map rm-community-in permit 30
    match community cme-prefmod-range
    call rm-prefmod
   route-map rm-community-in permit 40
   !
   ! #####################################################################
   ! Community actions to take when advertising a route.
   ! These are filtering route-maps,
   !
   ! Deny customer routes to upstream with cust-only set.
   route-map rm-community-filt-to-upstream deny 10
    match community cm-learnt-cust
    match community cm-cust-only
   route-map rm-community-filt-to-upstream permit 20
   !
   ! Deny customer routes to other customers with upstream-only set.
   route-map rm-community-filt-to-cust deny 10
    match community cm-learnt-cust
    match community cm-upstream-only
   route-map rm-community-filt-to-cust permit 20
   !
   ! ###################################################################
   ! The top-level route-maps applied to sessions. Further entries could
   ! be added obviously..
   !
   ! Customers
   route-map rm-cust-in permit 10
    call rm-community-in
    on-match next
   route-map rm-cust-in permit 20
    set community additive 64512:3100
   route-map rm-cust-in permit 30
   !
   route-map rm-cust-out permit 10
    call rm-community-filt-to-cust
    on-match next
   route-map rm-cust-out permit 20
   !
   ! Upstream transit ASes
   route-map rm-upstream-out permit 10
    description filter customer prefixes which are marked cust-only
    call rm-community-filt-to-upstream
    on-match next
   route-map rm-upstream-out permit 20
    description only customer routes are provided to upstreams/peers
    match community cm-learnt-cust
   !
   ! Peer ASes
   ! outbound policy is same as for upstream
   route-map rm-peer-out permit 10
    call rm-upstream-out
   !
   route-map rm-peer-in permit 10
    set community additive 64512:3200


Example of how to set up a 6-Bone connection.

.. code-block:: frr

   ! bgpd configuration
   ! ==================
   !
   ! MP-BGP configuration
   !
   router bgp 7675
    bgp router-id 10.0.0.1
    neighbor 3ffe:1cfa:0:2:2a0:c9ff:fe9e:f56 remote-as `as-number`
   !
    address-family ipv6
    network 3ffe:506::/32
    neighbor 3ffe:1cfa:0:2:2a0:c9ff:fe9e:f56 activate
    neighbor 3ffe:1cfa:0:2:2a0:c9ff:fe9e:f56 route-map set-nexthop out
    neighbor 3ffe:1cfa:0:2:2c0:4fff:fe68:a231 remote-as `as-number`
    neighbor 3ffe:1cfa:0:2:2c0:4fff:fe68:a231 route-map set-nexthop out
    exit-address-family
   !
   ipv6 access-list all permit any
   !
   ! Set output nexthop address.
   !
   route-map set-nexthop permit 10
    match ipv6 address all
    set ipv6 nexthop global 3ffe:1cfa:0:2:2c0:4fff:fe68:a225
    set ipv6 nexthop local fe80::2c0:4fff:fe68:a225
   !
   log file bgpd.log
   !

.. _bgp-tcp-mss:

BGP tcp-mss support
===================
TCP provides a mechanism for the user to specify the max segment size.
setsockopt API is used to set the max segment size for TCP session. We
can configure this as part of BGP neighbor configuration.

This document explains how to avoid ICMP vulnerability issues by limiting
TCP max segment size when you are using MTU discovery. Using MTU discovery
on TCP paths is one method of avoiding BGP packet fragmentation.

TCP negotiates a maximum segment size (MSS) value during session connection
establishment between two peers. The MSS value negotiated is primarily based
on the maximum transmission unit (MTU) of the interfaces to which the
communicating peers are directly connected. However, due to variations in
link MTU on the path taken by the TCP packets, some packets in the network
that are well within the MSS value might be fragmented when the packet size
exceeds the link's MTU.

This feature is supported with TCP over IPv4 and TCP over IPv6.

CLI Configuration:
------------------
Below configuration can be done in router bgp mode and allows the user to
configure the tcp-mss value per neighbor. The configuration gets applied
only after hard reset is performed on that neighbor. If we configure tcp-mss
on both the neighbors then both neighbors need to be reset.

The configuration takes effect based on below rules, so there is a configured
tcp-mss and a synced tcp-mss value per TCP session.

By default if the configuration is not done then the TCP max segment size is
set to the Maximum Transmission unit (MTU) – (IP/IP6 header size + TCP header
size + ethernet header). For IPv4 its MTU – (20 bytes IP header + 20 bytes TCP
header + 12 bytes ethernet header) and for IPv6 its MTU – (40 bytes IPv6 header
+ 20 bytes TCP header + 12 bytes ethernet header).

If the config is done then it reduces 12-14 bytes for the ether header and
uses it after synchronizing in TCP handshake.

.. clicmd:: neighbor <A.B.C.D|X:X::X:X|WORD> tcp-mss (1-65535)

When tcp-mss is configured kernel reduces 12-14 bytes for ethernet header.
E.g. if tcp-mss is configured as 150 the synced value will be 138.

Note: configured and synced value is different since TCP module will reduce
12 bytes for ethernet header.

Running config:
---------------

.. code-block:: frr

   frr# show running-config
   Building configuration...

   Current configuration:
   !
   router bgp 100
    bgp router-id 192.0.2.1
    neighbor 198.51.100.2 remote-as 100
    neighbor 198.51.100.2 tcp-mss 150       => new entry
    neighbor 2001:DB8::2 remote-as 100
    neighbor 2001:DB8::2 tcp-mss 400        => new entry

Show command:
-------------

.. code-block:: frr

   frr# show bgp neighbors 198.51.100.2
   BGP neighbor is 198.51.100.2, remote AS 100, local AS 100, internal link
   Hostname: frr
     BGP version 4, remote router ID 192.0.2.2, local router ID 192.0.2.1
     BGP state = Established, up for 02:15:28
     Last read 00:00:28, Last write 00:00:28
     Hold time is 180, keepalive interval is 60 seconds
     Configured tcp-mss is 150, synced tcp-mss is 138     => new display

.. code-block:: frr

   frr# show bgp neighbors 2001:DB8::2
   BGP neighbor is 2001:DB8::2, remote AS 100, local AS 100, internal link
   Hostname: frr
     BGP version 4, remote router ID 192.0.2.2, local router ID 192.0.2.1
     BGP state = Established, up for 02:16:34
     Last read 00:00:34, Last write 00:00:34
     Hold time is 180, keepalive interval is 60 seconds
     Configured tcp-mss is 400, synced tcp-mss is 388      => new display

Show command json output:
-------------------------

.. code-block:: frr

   frr# show bgp neighbors 2001:DB8::2 json
   {
     "2001:DB8::2":{
       "remoteAs":100,
       "localAs":100,
       "nbrInternalLink":true,
       "hostname":"frr",
       "bgpVersion":4,
       "remoteRouterId":"192.0.2.2",
       "localRouterId":"192.0.2.1",
       "bgpState":"Established",
       "bgpTimerUpMsec":8349000,
       "bgpTimerUpString":"02:19:09",
       "bgpTimerUpEstablishedEpoch":1613054251,
       "bgpTimerLastRead":9000,
       "bgpTimerLastWrite":9000,
       "bgpInUpdateElapsedTimeMsecs":8347000,
       "bgpTimerHoldTimeMsecs":180000,
       "bgpTimerKeepAliveIntervalMsecs":60000,
       "bgpTcpMssConfigured":400,                                   => new entry
       "bgpTcpMssSynced":388,                                  => new entry

.. code-block:: frr

   frr# show bgp neighbors 198.51.100.2 json
   {
     "198.51.100.2":{
       "remoteAs":100,
       "localAs":100,
       "nbrInternalLink":true,
       "hostname":"frr",
       "bgpVersion":4,
       "remoteRouterId":"192.0.2.2",
       "localRouterId":"192.0.2.1",
       "bgpState":"Established",
       "bgpTimerUpMsec":8370000,
       "bgpTimerUpString":"02:19:30",
       "bgpTimerUpEstablishedEpoch":1613054251,
       "bgpTimerLastRead":30000,
       "bgpTimerLastWrite":30000,
       "bgpInUpdateElapsedTimeMsecs":8368000,
       "bgpTimerHoldTimeMsecs":180000,
       "bgpTimerKeepAliveIntervalMsecs":60000,
       "bgpTcpMssConfigured":150,                                  => new entry
       "bgpTcpMssSynced":138,                                  => new entry

.. include:: routeserver.rst

.. include:: rpki.rst

.. include:: wecmp_linkbw.rst

.. include:: flowspec.rst

.. [#med-transitivity-rant] For some set of objects to have an order, there *must* be some binary ordering relation that is defined for *every* combination of those objects, and that relation *must* be transitive. I.e.:, if the relation operator is <, and if a < b and b < c then that relation must carry over and it *must* be that a < c for the objects to have an order. The ordering relation may allow for equality, i.e. a < b and b < a may both be true and imply that a and b are equal in the order and not distinguished by it, in which case the set has a partial order. Otherwise, if there is an order, all the objects have a distinct place in the order and the set has a total order)
.. [bgp-route-osci-cond] McPherson, D. and Gill, V. and Walton, D., "Border Gateway Protocol (BGP) Persistent Route Oscillation Condition", IETF RFC3345
.. [stable-flexible-ibgp] Flavel, A. and M. Roughan, "Stable and flexible iBGP", ACM SIGCOMM 2009
.. [ibgp-correctness] Griffin, T. and G. Wilfong, "On the correctness of IBGP configuration", ACM SIGCOMM 2002