[mirror_frr.git] / doc / developer / next-hop-tracking.rst

Next Hop Tracking
==================

Next hop tracking is an optimization feature that reduces the processing time
involved in the BGP bestpath algorithm by monitoring changes to the routing
table.

Background
-----------

Recursive routes are of the form:

::

    p/m --> n
    [Ex: 1.1.0.0/16 --> 2.2.2.2]

where 'n' itself is resolved through another route as follows:

::

    p2/m --> h, interface
    [Ex: 2.2.2.0/24 --> 3.3.3.3, eth0]

Usually, BGP routes are recursive in nature and BGP nexthops get resolved
through an IGP route. IGP usually adds its routes pointing to an interface
(these are called non-recursive routes).

When BGP receives a recursive route from a peer, it needs to validate the
nexthop. The path is marked valid or invalid based on the reachability status
of the nexthop. Nexthop validation is also important for BGP decision process
as the metric to reach the nexthop is a parameter to best path selection
process.

As it goes with routing, this is a dynamic process. Route to the nexthop can
change. The nexthop can become unreachable or reachable. In the current BGP
implementation, the nexthop validation is done periodically in the scanner run.
The default scanner run interval is one minute. Every minute, the scanner task
walks the entire BGP table. It checks the validity of each nexthop with Zebra
(the routing table manager) through a request and response message exchange
between BGP and Zebra process. BGP process is blocked for that duration. The
mechanism has two major drawbacks:

- The scanner task runs to completion. That can potentially starve the other
  tasks for long periods of time, based on the BGP table size and number of
  nexthops.

- Convergence around routing changes that affect the nexthops can be long
  (around a minute with the default intervals). The interval can be shortened
  to achieve faster reaction time, but it makes the first problem worse, with
  the scanner task consuming most of the CPU resources.

The next-hop tracking feature makes this process event-driven. It eliminates
periodic nexthop validation and introduces an asynchronous communication path
between BGP and Zebra for route change notifications that can then be acted
upon.

Goal
----

Stating the obvious, the main goal is to remove the two limitations we
discussed in the previous section. The goals, in a constructive tone,
are the following:

- **Fairness**: the scanner run should not consume an unjustly high amount of
  CPU time. This should give an overall good performance and response time to
  other events (route changes, session events, IO/user interface).

- **Convergence**: BGP must react to nexthop changes instantly and provide
  sub-second convergence. This may involve diverting the routes from one
  nexthop to another.

Overview of changes
------------------------

The changes are in both BGP and Zebra modules.  The short summary is
the following:

- Zebra implements a registration mechanism by which clients can
  register for next hop notification. Consequently, it maintains a
  separate table, per (VRF, AF) pair, of next hops and interested
  client-list per next hop.

- When the main routing table changes in Zebra, it evaluates the next
  hop table: for each next hop, it checks if the route table
  modifications have changed its state. If so, it notifies the
  interested clients.

- BGP is one such client. It registers the next hops corresponding to
  all of its received routes/paths. It also threads the paths against
  each nexthop structure.

- When BGP receives a next hop notification from Zebra, it walks the
  corresponding path list. It makes them valid or invalid depending
  on the next hop notification. It then re-computes best path for the
  corresponding destination. This may result in re-announcing those
  destinations to peers.

Design
------

Modules
^^^^^^^

The core design introduces an "nht" (next hop tracking) module in BGP
and "rnh" (recursive nexthop) module in Zebra. The "nht" module
provides the following APIs:

+----------------------------+--------------------------------------------------+
| Function                   | Action                                           |
+============================+==================================================+
| bgp_find_or_add_nexthop()  | find or add a nexthop in BGP nexthop table       |
+----------------------------+--------------------------------------------------+
| bgp_parse_nexthop_update() | parse a nexthop update message coming from zebra |
+----------------------------+--------------------------------------------------+

The "rnh" module provides the following APIs:

+----------------------------+----------------------------------------------------------------------------------------------------------+
| Function                   | Action                                                                                                   |
+============================+==========================================================================================================+
| zebra_add_rnh()            | add a recursive nexthop                                                                                  |
+----------------------------+----------------------------------------------------------------------------------------------------------+
| zebra_delete_rnh()         | delete a recursive nexthop                                                                               |
+----------------------------+----------------------------------------------------------------------------------------------------------+
| zebra_lookup_rnh()         | lookup a recursive nexthop                                                                               |
+----------------------------+----------------------------------------------------------------------------------------------------------+
| zebra_add_rnh_client()     | register a client for nexthop notifications against a recursive nexthop                                  |
+----------------------------+----------------------------------------------------------------------------------------------------------+
| zebra_remove_rnh_client()  | remove the client registration for a recursive nexthop                                                   |
+----------------------------+----------------------------------------------------------------------------------------------------------+
| zebra_evaluate_rnh_table() | (re)evaluate the recursive nexthop table (most probably because the main routing table has changed).     |
+----------------------------+----------------------------------------------------------------------------------------------------------+
| zebra_cleanup_rnh_client() | Cleanup a client from the "rnh" module data structures (most probably because the client is going away). |
+----------------------------+----------------------------------------------------------------------------------------------------------+

4.2. Control flow

The next hop registration control flow is the following:

::

    <====      BGP Process       ====>|<====      Zebra Process      ====>
                                      |
    receive module     nht module     |  zserv module        rnh module
    ----------------------------------------------------------------------
                  |                   |                  |
    bgp_update_   |                   |                  |
          main()  | bgp_find_or_add_  |                  |
                  |        nexthop()  |                  |
                  |                   |                  |
                  |                   | zserv_nexthop_   |
                  |                   |       register() |
                  |                   |                  | zebra_add_rnh()
                  |                   |                  |


The next hop notification control flow is the following:

::

    <====     Zebra Process    ====>|<====      BGP Process       ====>
                                    |
    rib module         rnh module   |     zebra module        nht module
    ----------------------------------------------------------------------
                  |                 |                   |
    meta_queue_   |                 |                   |
        process() | zebra_evaluate_ |                   |
                  |     rnh_table() |                   |
                  |                 |                   |
                  |                 | bgp_read_nexthop_ |
                  |                 |          update() |
                  |                 |                   | bgp_parse_
                  |                 |                   | nexthop_update()
                  |                 |                   |


zclient message format
^^^^^^^^^^^^^^^^^^^^^^

ZEBRA_NEXTHOP_REGISTER and ZEBRA_NEXTHOP_UNREGISTER messages are
encoded in the following way:

::

    .   0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     AF                        |  prefix len   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .      Nexthop prefix                                           .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     AF                        |  prefix len   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .      Nexthop prefix                                           .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


``ZEBRA_NEXTHOP_UPDATE`` message is encoded as follows:

::

    .   0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     AF                        |  prefix len   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .      Nexthop prefix getting resolved                          .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        metric                                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  #nexthops    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | nexthop type  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .      resolving Nexthop details                                .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | nexthop type  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .      resolving Nexthop details                                .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


BGP data structure
^^^^^^^^^^^^^^^^^^
Legend:

::

    /\   struct bgp_node: a BGP destination/route/prefix
    \/

    [ ]  struct bgp_path_info: a BGP path (e.g. route received from a peer)

     _
    (_)  struct bgp_nexthop_cache: a BGP nexthop

    /\         NULL
    \/--+        ^
        |        :
        +--[ ]--[ ]--[ ]--> NULL
    /\           :
    \/--+        :
        |        :
        +--[ ]--[ ]--> NULL
                 :
     _           :
    (_)...........


Zebra data structure
^^^^^^^^^^^^^^^^^^^^

RNH table::

   .  O
     / \
    O   O
       / \
      O   O

   struct rnh
   {
     uint8_t flags;
     struct route_entry *state;
     struct list *client_list;
     struct route_node *node;
   };

User interface changes
^^^^^^^^^^^^^^^^^^^^^^

::

    frr# show ip nht
    3.3.3.3
     resolved via kernel
     via 11.0.0.6, swp1
     Client list: bgp(fd 12)
    11.0.0.10
     resolved via connected
     is directly connected, swp2
     Client list: bgp(fd 12)
    11.0.0.18
     resolved via connected
     is directly connected, swp4
     Client list: bgp(fd 12)
    11.11.11.11
     resolved via kernel
     via 10.0.1.2, eth0
     Client list: bgp(fd 12)

    frr# show ip bgp nexthop
    Current BGP nexthop cache:
     3.3.3.3 valid [IGP metric 0], #paths 3
      Last update: Wed Oct 16 04:43:49 2013

     11.0.0.10 valid [IGP metric 1], #paths 1
      Last update: Wed Oct 16 04:43:51 2013

     11.0.0.18 valid [IGP metric 1], #paths 2
      Last update: Wed Oct 16 04:43:47 2013

     11.11.11.11 valid [IGP metric 0], #paths 1
      Last update: Wed Oct 16 04:43:47 2013

    frr# show ipv6 nht
    frr# show ip bgp nexthop detail

    frr# debug bgp nht
    frr# debug zebra nht

    6. Sample test cases

         r2----r3
        /  \  /
      r1----r4

    - Verify that a change in IGP cost triggers NHT
      + shutdown the r1-r4 and r2-r4 links
      + no shut the r1-r4 and r2-r4 links and wait for OSPF to come back
        up
      + We should be back to the original nexthop via r4 now
    - Verify that a NH becoming unreachable triggers NHT
      + Shutdown all links to r4
    - Verify that a NH becoming reachable triggers NHT
      + no shut all links to r4

Future work
^^^^^^^^^^^

- route-policy for next hop validation (e.g. ignore default route)
- damping for rapid next hop changes
- prioritized handling of nexthop changes ((un)reachability vs. metric
  changes)
- handling recursion loop, e.g::

   11.11.11.11/32 -> 12.12.12.12
   12.12.12.12/32 -> 11.11.11.11
   11.0.0.0/8 -> <interface>
- better statistics
Commit	Line	Data
b22ba015 QY	1	Next Hop Tracking
	2	==================
	3
	4	Next hop tracking is an optimization feature that reduces the processing time
	5	involved in the BGP bestpath algorithm by monitoring changes to the routing
	6	table.
	7
	8	Background
	9	-----------
	10
	11	Recursive routes are of the form:
	12
	13	::
	14
	15	p/m --> n
	16	[Ex: 1.1.0.0/16 --> 2.2.2.2]
	17
	18	where 'n' itself is resolved through another route as follows:
	19
	20	::
	21
	22	p2/m --> h, interface
	23	[Ex: 2.2.2.0/24 --> 3.3.3.3, eth0]
	24
	25	Usually, BGP routes are recursive in nature and BGP nexthops get resolved
	26	through an IGP route. IGP usually adds its routes pointing to an interface
	27	(these are called non-recursive routes).
	28
	29	When BGP receives a recursive route from a peer, it needs to validate the
	30	nexthop. The path is marked valid or invalid based on the reachability status
	31	of the nexthop. Nexthop validation is also important for BGP decision process
	32	as the metric to reach the nexthop is a parameter to best path selection
	33	process.
	34
	35	As it goes with routing, this is a dynamic process. Route to the nexthop can
	36	change. The nexthop can become unreachable or reachable. In the current BGP
	37	implementation, the nexthop validation is done periodically in the scanner run.
	38	The default scanner run interval is one minute. Every minute, the scanner task
	39	walks the entire BGP table. It checks the validity of each nexthop with Zebra
	40	(the routing table manager) through a request and response message exchange
	41	between BGP and Zebra process. BGP process is blocked for that duration. The
	42	mechanism has two major drawbacks:
	43
	44	- The scanner task runs to completion. That can potentially starve the other
	45	tasks for long periods of time, based on the BGP table size and number of
	46	nexthops.
	47
	48	- Convergence around routing changes that affect the nexthops can be long
	49	(around a minute with the default intervals). The interval can be shortened
	50	to achieve faster reaction time, but it makes the first problem worse, with
	51	the scanner task consuming most of the CPU resources.
	52
	53	The next-hop tracking feature makes this process event-driven. It eliminates
	54	periodic nexthop validation and introduces an asynchronous communication path
	55	between BGP and Zebra for route change notifications that can then be acted
	56	upon.
	57
	58	Goal
	59	----
	60
	61	Stating the obvious, the main goal is to remove the two limitations we
	62	discussed in the previous section. The goals, in a constructive tone,
	63	are the following:
	64
65	- Fairness: the scanner run should not consume an unjustly high amount of
66	CPU time. This should give an overall good performance and response time to
67	other events (route changes, session events, IO/user interface).
68
69	- Convergence: BGP must react to nexthop changes instantly and provide
70	sub-second convergence. This may involve diverting the routes from one
71	nexthop to another.
72
73	Overview of changes
74	------------------------
75
76	The changes are in both BGP and Zebra modules. The short summary is
77	the following:
78
79	- Zebra implements a registration mechanism by which clients can
80	register for next hop notification. Consequently, it maintains a
81	separate table, per (VRF, AF) pair, of next hops and interested
82	client-list per next hop.
83
84	- When the main routing table changes in Zebra, it evaluates the next
85	hop table: for each next hop, it checks if the route table
86	modifications have changed its state. If so, it notifies the
87	interested clients.
88
89	- BGP is one such client. It registers the next hops corresponding to
90	all of its received routes/paths. It also threads the paths against
91	each nexthop structure.
92
93	- When BGP receives a next hop notification from Zebra, it walks the
94	corresponding path list. It makes them valid or invalid depending
95	on the next hop notification. It then re-computes best path for the
96	corresponding destination. This may result in re-announcing those
97	destinations to peers.
98
99	Design
100	------
101
102	Modules
75ca3b11	103	^^^^^^^
b22ba015 QY	104
	105	The core design introduces an "nht" (next hop tracking) module in BGP
	106	and "rnh" (recursive nexthop) module in Zebra. The "nht" module
	107	provides the following APIs:
	108
	109	+----------------------------+--------------------------------------------------+
	110	\| Function \| Action \|
	111	+============================+==================================================+
	112	\| bgp_find_or_add_nexthop() \| find or add a nexthop in BGP nexthop table \|
	113	+----------------------------+--------------------------------------------------+
b22ba015 QY	114	\| bgp_parse_nexthop_update() \| parse a nexthop update message coming from zebra \|
	115	+----------------------------+--------------------------------------------------+
	116
	117	The "rnh" module provides the following APIs:
	118
	119	+----------------------------+----------------------------------------------------------------------------------------------------------+
	120	\| Function \| Action \|
	121	+============================+==========================================================================================================+
	122	\| zebra_add_rnh() \| add a recursive nexthop \|
	123	+----------------------------+----------------------------------------------------------------------------------------------------------+
	124	\| zebra_delete_rnh() \| delete a recursive nexthop \|
	125	+----------------------------+----------------------------------------------------------------------------------------------------------+
	126	\| zebra_lookup_rnh() \| lookup a recursive nexthop \|
	127	+----------------------------+----------------------------------------------------------------------------------------------------------+
	128	\| zebra_add_rnh_client() \| register a client for nexthop notifications against a recursive nexthop \|
	129	+----------------------------+----------------------------------------------------------------------------------------------------------+
	130	\| zebra_remove_rnh_client() \| remove the client registration for a recursive nexthop \|
	131	+----------------------------+----------------------------------------------------------------------------------------------------------+
	132	\| zebra_evaluate_rnh_table() \| (re)evaluate the recursive nexthop table (most probably because the main routing table has changed). \|
	133	+----------------------------+----------------------------------------------------------------------------------------------------------+
	134	\| zebra_cleanup_rnh_client() \| Cleanup a client from the "rnh" module data structures (most probably because the client is going away). \|
	135	+----------------------------+----------------------------------------------------------------------------------------------------------+
	136
	137	4.2. Control flow
	138
	139	The next hop registration control flow is the following:
	140
	141	::
	142
	143	<==== BGP Process ====>\|<==== Zebra Process ====>
	144	\|
	145	receive module nht module \| zserv module rnh module
	146	----------------------------------------------------------------------
	147	\| \| \|
	148	bgp_update_ \| \| \|
	149	main() \| bgp_find_or_add_ \| \|
	150	\| nexthop() \| \|
	151	\| \| \|
	152	\| \| zserv_nexthop_ \|
	153	\| \| register() \|
	154	\| \| \| zebra_add_rnh()
	155	\| \| \|
76bd1499	156
b22ba015 QY	157
	158	The next hop notification control flow is the following:
	159
	160	::
	161
	162	<==== Zebra Process ====>\|<==== BGP Process ====>
	163	\|
	164	rib module rnh module \| zebra module nht module
	165	----------------------------------------------------------------------
	166	\| \| \|
	167	meta_queue_ \| \| \|
	168	process() \| zebra_evaluate_ \| \|
	169	\| rnh_table() \| \|
	170	\| \| \|
	171	\| \| bgp_read_nexthop_ \|
	172	\| \| update() \|
	173	\| \| \| bgp_parse_
	174	\| \| \| nexthop_update()
	175	\| \| \|
	176
	177
	178	zclient message format
75ca3b11	179	^^^^^^^^^^^^^^^^^^^^^^
b22ba015 QY	180
	181	ZEBRA_NEXTHOP_REGISTER and ZEBRA_NEXTHOP_UNREGISTER messages are
	182	encoded in the following way:
	183
	184	::
	185
	186	. 0 1 2 3
	187	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	188	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	189	\| AF \| prefix len \|
	190	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	191	. Nexthop prefix .
	192	. .
	193	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	194	. .
	195	. .
	196	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	197	\| AF \| prefix len \|
	198	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	199	. Nexthop prefix .
	200	. .
	201	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
76bd1499	202
b22ba015 QY	203
	204	``ZEBRA_NEXTHOP_UPDATE`` message is encoded as follows:
	205
	206	::
	207
	208	. 0 1 2 3
	209	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	210	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	211	\| AF \| prefix len \|
	212	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	213	. Nexthop prefix getting resolved .
	214	. .
	215	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	216	\| metric \|
	217	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	218	\| #nexthops \|
	219	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	220	\| nexthop type \|
	221	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	222	. resolving Nexthop details .
	223	. .
	224	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	225	. .
	226	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	227	\| nexthop type \|
	228	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	229	. resolving Nexthop details .
	230	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	231
	232
	233	BGP data structure
75ca3b11	234	^^^^^^^^^^^^^^^^^^
b22ba015 QY	235	Legend:
	236
	237	::
	238
	239	/\ struct bgp_node: a BGP destination/route/prefix
	240	\/
76bd1499	241
4b7e6066	242	[ ] struct bgp_path_info: a BGP path (e.g. route received from a peer)
76bd1499	243
b22ba015 QY	244	_
b22ba015 QY	245	(_) struct bgp_nexthop_cache: a BGP nexthop
76bd1499	246
b22ba015 QY	247	/\ NULL
	248	\/--+ ^
	249	\| :
	250	+--[ ]--[ ]--[ ]--> NULL
	251	/\ :
	252	\/--+ :
	253	\| :
	254	+--[ ]--[ ]--> NULL
	255	:
	256	_ :
	257	(_)...........
	258
	259
	260	Zebra data structure
75ca3b11	261	^^^^^^^^^^^^^^^^^^^^
b22ba015 QY	262
	263	RNH table::
	264
d7c0a89a QY	265	. O
	266	/ \
	267	O O
	268	/ \
	269	O O
4c97fd1a	270
d7c0a89a QY	271	struct rnh
	272	{
	273	uint8_t flags;
	274	struct route_entry *state;
	275	struct list *client_list;
	276	struct route_node *node;
	277	};
b22ba015 QY	278
b22ba015 QY	279	User interface changes
75ca3b11	280	^^^^^^^^^^^^^^^^^^^^^^
b22ba015 QY	281
	282	::
	283
	284	frr# show ip nht
	285	3.3.3.3
	286	resolved via kernel
	287	via 11.0.0.6, swp1
	288	Client list: bgp(fd 12)
	289	11.0.0.10
	290	resolved via connected
	291	is directly connected, swp2
	292	Client list: bgp(fd 12)
	293	11.0.0.18
	294	resolved via connected
	295	is directly connected, swp4
	296	Client list: bgp(fd 12)
	297	11.11.11.11
	298	resolved via kernel
	299	via 10.0.1.2, eth0
	300	Client list: bgp(fd 12)
76bd1499	301
b22ba015 QY	302	frr# show ip bgp nexthop
	303	Current BGP nexthop cache:
	304	3.3.3.3 valid [IGP metric 0], #paths 3
	305	Last update: Wed Oct 16 04:43:49 2013
76bd1499	306
b22ba015 QY	307	11.0.0.10 valid [IGP metric 1], #paths 1
b22ba015 QY	308	Last update: Wed Oct 16 04:43:51 2013
76bd1499	309
b22ba015 QY	310	11.0.0.18 valid [IGP metric 1], #paths 2
b22ba015 QY	311	Last update: Wed Oct 16 04:43:47 2013
76bd1499	312
b22ba015 QY	313	11.11.11.11 valid [IGP metric 0], #paths 1
b22ba015 QY	314	Last update: Wed Oct 16 04:43:47 2013
76bd1499	315
b22ba015 QY	316	frr# show ipv6 nht
b22ba015 QY	317	frr# show ip bgp nexthop detail
76bd1499	318
b22ba015 QY	319	frr# debug bgp nht
b22ba015 QY	320	frr# debug zebra nht
76bd1499	321
b22ba015	322	6. Sample test cases
76bd1499	323
b22ba015 QY	324	r2----r3
	325	/ \ /
	326	r1----r4
76bd1499	327
b22ba015 QY	328	- Verify that a change in IGP cost triggers NHT
	329	+ shutdown the r1-r4 and r2-r4 links
	330	+ no shut the r1-r4 and r2-r4 links and wait for OSPF to come back
	331	up
	332	+ We should be back to the original nexthop via r4 now
	333	- Verify that a NH becoming unreachable triggers NHT
	334	+ Shutdown all links to r4
	335	- Verify that a NH becoming reachable triggers NHT
	336	+ no shut all links to r4
	337
	338	Future work
75ca3b11	339	^^^^^^^^^^^
b22ba015 QY	340
	341	- route-policy for next hop validation (e.g. ignore default route)
	342	- damping for rapid next hop changes
	343	- prioritized handling of nexthop changes ((un)reachability vs. metric
	344	changes)
	345	- handling recursion loop, e.g::
	346
	347	11.11.11.11/32 -> 12.12.12.12
	348	12.12.12.12/32 -> 11.11.11.11
	349	11.0.0.0/8 -> <interface>
	350	- better statistics