FAQ: Describe why OVS can't prepopulate the kernel flow table.

[mirror_ovs.git] / FAQ.md

Frequently Asked Questions
==========================

Open vSwitch <http://openvswitch.org>

General
-------

### Q: What is Open vSwitch?

A: Open vSwitch is a production quality open source software switch
   designed to be used as a vswitch in virtualized server
   environments.  A vswitch forwards traffic between different VMs on
   the same physical host and also forwards traffic between VMs and
   the physical network.  Open vSwitch supports standard management
   interfaces (e.g. sFlow, NetFlow, IPFIX, RSPAN, CLI), and is open to
   programmatic extension and control using OpenFlow and the OVSDB
   management protocol.

   Open vSwitch as designed to be compatible with modern switching
   chipsets.  This means that it can be ported to existing high-fanout
   switches allowing the same flexible control of the physical
   infrastructure as the virtual infrastructure.  It also means that
   Open vSwitch will be able to take advantage of on-NIC switching
   chipsets as their functionality matures.

### Q: What virtualization platforms can use Open vSwitch?

A: Open vSwitch can currently run on any Linux-based virtualization
   platform (kernel 2.6.32 and newer), including: KVM, VirtualBox, Xen,
   Xen Cloud Platform, XenServer. As of Linux 3.3 it is part of the
   mainline kernel.  The bulk of the code is written in platform-
   independent C and is easily ported to other environments.  We welcome
   inquires about integrating Open vSwitch with other virtualization
   platforms.

### Q: How can I try Open vSwitch?

A: The Open vSwitch source code can be built on a Linux system.  You can
   build and experiment with Open vSwitch on any Linux machine.
   Packages for various Linux distributions are available on many
   platforms, including: Debian, Ubuntu, Fedora.

   You may also download and run a virtualization platform that already
   has Open vSwitch integrated.  For example, download a recent ISO for
   XenServer or Xen Cloud Platform.  Be aware that the version
   integrated with a particular platform may not be the most recent Open
   vSwitch release.

### Q: Does Open vSwitch only work on Linux?

A: No, Open vSwitch has been ported to a number of different operating
   systems and hardware platforms.  Most of the development work occurs
   on Linux, but the code should be portable to any POSIX system.  We've
   seen Open vSwitch ported to a number of different platforms,
   including FreeBSD, Windows, and even non-POSIX embedded systems.

   By definition, the Open vSwitch Linux kernel module only works on
   Linux and will provide the highest performance.  However, a userspace
   datapath is available that should be very portable.

### Q: What's involved with porting Open vSwitch to a new platform or switching ASIC?

A: The [PORTING.md] document describes how one would go about
   porting Open vSwitch to a new operating system or hardware platform.

### Q: Why would I use Open vSwitch instead of the Linux bridge?

A: Open vSwitch is specially designed to make it easier to manage VM
   network configuration and monitor state spread across many physical
   hosts in dynamic virtualized environments.  Please see
   [WHY-OVS.md] for a more detailed description of how Open vSwitch
   relates to the Linux Bridge.

### Q: How is Open vSwitch related to distributed virtual switches like the VMware vNetwork distributed switch or the Cisco Nexus 1000V?

A: Distributed vswitch applications (e.g., VMware vNetwork distributed
   switch, Cisco Nexus 1000V) provide a centralized way to configure and
   monitor the network state of VMs that are spread across many physical
   hosts.  Open vSwitch is not a distributed vswitch itself, rather it
   runs on each physical host and supports remote management in a way
   that makes it easier for developers of virtualization/cloud
   management platforms to offer distributed vswitch capabilities.

   To aid in distribution, Open vSwitch provides two open protocols that
   are specially designed for remote management in virtualized network
   environments: OpenFlow, which exposes flow-based forwarding state,
   and the OVSDB management protocol, which exposes switch port state.
   In addition to the switch implementation itself, Open vSwitch
   includes tools (ovs-ofctl, ovs-vsctl) that developers can script and
   extend to provide distributed vswitch capabilities that are closely
   integrated with their virtualization management platform.

### Q: Why doesn't Open vSwitch support distribution?

A: Open vSwitch is intended to be a useful component for building
   flexible network infrastructure. There are many different approaches
   to distribution which balance trade-offs between simplicity,
   scalability, hardware compatibility, convergence times, logical
   forwarding model, etc. The goal of Open vSwitch is to be able to
   support all as a primitive building block rather than choose a
   particular point in the distributed design space.

### Q: How can I contribute to the Open vSwitch Community?

A: You can start by joining the mailing lists and helping to answer
   questions.  You can also suggest improvements to documentation.  If
   you have a feature or bug you would like to work on, send a mail to
   one of the mailing lists:

   http://openvswitch.org/mlists/

### Q: Why can I no longer connect to my OpenFlow controller or OVSDB manager?

A: Starting in OVS 2.4, we switched the default ports to the
   IANA-specified port numbers for OpenFlow (6633->6653) and OVSDB
   (6632->6640).  We recommend using these port numbers, but if you
   cannot, all the programs allow overriding the default port.  See the
   appropriate man page.


Releases
--------

### Q: What does it mean for an Open vSwitch release to be LTS (long-term support)?

A: All official releases have been through a comprehensive testing
   process and are suitable for production use.  Planned releases will
   occur several times a year.  If a significant bug is identified in an
   LTS release, we will provide an updated release that includes the
   fix.  Releases that are not LTS may not be fixed and may just be
   supplanted by the next major release.  The current LTS release is
   2.3.x.

### Q: What Linux kernel versions does each Open vSwitch release work with?

A: The following table lists the Linux kernel versions against which the
   given versions of the Open vSwitch kernel module will successfully
   build.  The Linux kernel versions are upstream kernel versions, so
   Linux kernels modified from the upstream sources may not build in
   some cases even if they are based on a supported version.  This is
   most notably true of Red Hat Enterprise Linux (RHEL) kernels, which
   are extensively modified from upstream.

| Open vSwitch | Linux kernel
|:------------:|:-------------:
|    1.4.x     | 2.6.18 to 3.2
|    1.5.x     | 2.6.18 to 3.2
|    1.6.x     | 2.6.18 to 3.2
|    1.7.x     | 2.6.18 to 3.3
|    1.8.x     | 2.6.18 to 3.4
|    1.9.x     | 2.6.18 to 3.8
|    1.10.x    | 2.6.18 to 3.8
|    1.11.x    | 2.6.18 to 3.8
|    2.0.x     | 2.6.32 to 3.10
|    2.1.x     | 2.6.32 to 3.11
|    2.3.x     | 2.6.32 to 3.14
|    2.4.x     | 2.6.32 to 4.0
|    2.5.x     | 2.6.32 to 4.2

   Open vSwitch userspace should also work with the Linux kernel module
   built into Linux 3.3 and later.

   Open vSwitch userspace is not sensitive to the Linux kernel version.
   It should build against almost any kernel, certainly against 2.6.32
   and later.

### Q: I get an error like this when I configure Open vSwitch:

       configure: error: Linux kernel in <dir> is version <x>, but
       version newer than <y> is not supported (please refer to the
       FAQ for advice)

   What should I do?

A: You have the following options:

   - Use the Linux kernel module supplied with the kernel that you are
     using.  (See also the following FAQ.)

   - If there is a newer released version of Open vSwitch, consider
     building that one, because it may support the kernel that you are
     building against.  (To find out, consult the table in the
     previous FAQ.)

   - The Open vSwitch "master" branch may support the kernel that you
     are using, so consider building the kernel module from "master".

  All versions of Open vSwitch userspace are compatible with all
  versions of the Open vSwitch kernel module, so you do not have to
  use the kernel module from one source along with the userspace
  programs from the same source.

### Q: What features are not available in the Open vSwitch kernel datapath that ships as part of the upstream Linux kernel?

A: The kernel module in upstream Linux does not include support for
   LISP. Work is in progress to add support for LISP to the upstream
   Linux version of the Open vSwitch kernel module. For now, if you
   need this feature, use the kernel module from the Open vSwitch
   distribution instead of the upstream Linux kernel module.

   Certain features require kernel support to function or to have
   reasonable performance. If the ovs-vswitchd log file indicates that
   a feature is not supported, consider upgrading to a newer upstream
   Linux release or using the kernel module paired with the userspace
   distribution.

### Q: Why do tunnels not work when using a kernel module other than the one packaged with Open vSwitch?

A: Support for tunnels was added to the upstream Linux kernel module
   after the rest of Open vSwitch. As a result, some kernels may contain
   support for Open vSwitch but not tunnels. The minimum kernel version
   that supports each tunnel protocol is:

| Protocol |  Linux Kernel
|:--------:|:-------------:
| GRE      |    3.11
| VXLAN    |    3.12
| Geneve   |    3.18
| LISP     | <not upstream>
| STT      | <not upstream>

   If you are using a version of the kernel that is older than the one
   listed above, it is still possible to use that tunnel protocol. However,
   you must compile and install the kernel module included with the Open
   vSwitch distribution rather than the one on your machine. If problems
   persist after doing this, check to make sure that the module that is
   loaded is the one you expect.

### Q: Why are UDP tunnel checksums not computed for VXLAN or Geneve?

A: Generating outer UDP checksums requires kernel support that was not
   part of the initial implementation of these protocols. If using the
   upstream Linux Open vSwitch module, you must use kernel 4.0 or
   newer. The out-of-tree modules from Open vSwitch release 2.4 and later
   support UDP checksums.

### Q: What features are not available when using the userspace datapath?

A: Tunnel virtual ports are not supported, as described in the
   previous answer.  It is also not possible to use queue-related
   actions.  On Linux kernels before 2.6.39, maximum-sized VLAN packets
   may not be transmitted.

### Q: What Linux kernel versions does IPFIX flow monitoring work with?

A: IPFIX flow monitoring requires the Linux kernel module from Linux
   3.10 or later, or the out-of-tree module from Open vSwitch version
   1.10.90 or later.

### Q: Should userspace or kernel be upgraded first to minimize downtime?

   In general, the Open vSwitch userspace should be used with the
   kernel version included in the same release or with the version
   from upstream Linux.  However, when upgrading between two releases
   of Open vSwitch it is best to migrate userspace first to reduce
   the possibility of incompatibilities.

### Q: What happened to the bridge compatibility feature?

A: Bridge compatibility was a feature of Open vSwitch 1.9 and earlier.
   When it was enabled, Open vSwitch imitated the interface of the
   Linux kernel "bridge" module.  This allowed users to drop Open
   vSwitch into environments designed to use the Linux kernel bridge
   module without adapting the environment to use Open vSwitch.

   Open vSwitch 1.10 and later do not support bridge compatibility.
   The feature was dropped because version 1.10 adopted a new internal
   architecture that made bridge compatibility difficult to maintain.
   Now that many environments use OVS directly, it would be rarely
   useful in any case.

   To use bridge compatibility, install OVS 1.9 or earlier, including
   the accompanying kernel modules (both the main and bridge
   compatibility modules), following the instructions that come with
   the release.  Be sure to start the ovs-brcompatd daemon.


Terminology
-----------

### Q: I thought Open vSwitch was a virtual Ethernet switch, but the documentation keeps talking about bridges.  What's a bridge?

A: In networking, the terms "bridge" and "switch" are synonyms.  Open
   vSwitch implements an Ethernet switch, which means that it is also
   an Ethernet bridge.

### Q: What's a VLAN?

A: See the "VLAN" section below.


Basic Configuration
-------------------

### Q: How do I configure a port as an access port?

A: Add "tag=VLAN" to your "ovs-vsctl add-port" command.  For example,
   the following commands configure br0 with eth0 as a trunk port (the
   default) and tap0 as an access port for VLAN 9:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0 tag=9

   If you want to configure an already added port as an access port,
   use "ovs-vsctl set", e.g.:

       ovs-vsctl set port tap0 tag=9

### Q: How do I configure a port as a SPAN port, that is, enable mirroring of all traffic to that port?

A: The following commands configure br0 with eth0 and tap0 as trunk
   ports.  All traffic coming in or going out on eth0 or tap0 is also
   mirrored to tap1; any traffic arriving on tap1 is dropped:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0
       ovs-vsctl add-port br0 tap1 \
           -- --id=@p get port tap1 \
           -- --id=@m create mirror name=m0 select-all=true output-port=@p \
           -- set bridge br0 mirrors=@m

   To later disable mirroring, run:

       ovs-vsctl clear bridge br0 mirrors

### Q: Does Open vSwitch support configuring a port in promiscuous mode?

A: Yes.  How you configure it depends on what you mean by "promiscuous
   mode":

  - Conventionally, "promiscuous mode" is a feature of a network
    interface card.  Ordinarily, a NIC passes to the CPU only the
    packets actually destined to its host machine.  It discards
    the rest to avoid wasting memory and CPU cycles.  When
    promiscuous mode is enabled, however, it passes every packet
    to the CPU.  On an old-style shared-media or hub-based
    network, this allows the host to spy on all packets on the
    network.  But in the switched networks that are almost
    everywhere these days, promiscuous mode doesn't have much
    effect, because few packets not destined to a host are
    delivered to the host's NIC.

    This form of promiscuous mode is configured in the guest OS of
    the VMs on your bridge, e.g. with "ifconfig".

  - The VMware vSwitch uses a different definition of "promiscuous
    mode".  When you configure promiscuous mode on a VMware vNIC,
    the vSwitch sends a copy of every packet received by the
    vSwitch to that vNIC.  That has a much bigger effect than just
    enabling promiscuous mode in a guest OS.  Rather than getting
    a few stray packets for which the switch does not yet know the
    correct destination, the vNIC gets every packet.  The effect
    is similar to replacing the vSwitch by a virtual hub.

    This "promiscuous mode" is what switches normally call "port
    mirroring" or "SPAN".  For information on how to configure
    SPAN, see "How do I configure a port as a SPAN port, that is,
    enable mirroring of all traffic to that port?"

### Q: How do I configure a DPDK port as an access port?

A: Firstly, you must have a DPDK-enabled version of Open vSwitch.

   If your version is DPDK-enabled it will support the --dpdk
   argument on the command line and will display lines with
   "EAL:..." during startup when --dpdk is supplied.

   Secondly, when adding a DPDK port, unlike a system port, the
   type for the interface must be specified. For example;

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 dpdk0 -- set Interface dpdk0 type=dpdk

   Finally, it is required that DPDK port names begin with 'dpdk'.

   See [INSTALL.DPDK.md] for more information on enabling and using DPDK with
   Open vSwitch.

### Q: How do I configure a VLAN as an RSPAN VLAN, that is, enable mirroring of all traffic to that VLAN?

A: The following commands configure br0 with eth0 as a trunk port and
   tap0 as an access port for VLAN 10.  All traffic coming in or going
   out on tap0, as well as traffic coming in or going out on eth0 in
   VLAN 10, is also mirrored to VLAN 15 on eth0.  The original tag for
   VLAN 10, in cases where one is present, is dropped as part of
   mirroring:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0 tag=10
       ovs-vsctl \
           -- --id=@m create mirror name=m0 select-all=true select-vlan=10 \
                                    output-vlan=15 \
           -- set bridge br0 mirrors=@m

   To later disable mirroring, run:

       ovs-vsctl clear bridge br0 mirrors

   Mirroring to a VLAN can disrupt a network that contains unmanaged
   switches.  See ovs-vswitchd.conf.db(5) for details.  Mirroring to a
   GRE tunnel has fewer caveats than mirroring to a VLAN and should
   generally be preferred.

### Q: Can I mirror more than one input VLAN to an RSPAN VLAN?

A: Yes, but mirroring to a VLAN strips the original VLAN tag in favor
   of the specified output-vlan.  This loss of information may make
   the mirrored traffic too hard to interpret.

   To mirror multiple VLANs, use the commands above, but specify a
   comma-separated list of VLANs as the value for select-vlan.  To
   mirror every VLAN, use the commands above, but omit select-vlan and
   its value entirely.

   When a packet arrives on a VLAN that is used as a mirror output
   VLAN, the mirror is disregarded.  Instead, in standalone mode, OVS
   floods the packet across all the ports for which the mirror output
   VLAN is configured.  (If an OpenFlow controller is in use, then it
   can override this behavior through the flow table.)  If OVS is used
   as an intermediate switch, rather than an edge switch, this ensures
   that the RSPAN traffic is distributed through the network.

   Mirroring to a VLAN can disrupt a network that contains unmanaged
   switches.  See ovs-vswitchd.conf.db(5) for details.  Mirroring to a
   GRE tunnel has fewer caveats than mirroring to a VLAN and should
   generally be preferred.

### Q: How do I configure mirroring of all traffic to a GRE tunnel?

A: The following commands configure br0 with eth0 and tap0 as trunk
   ports.  All traffic coming in or going out on eth0 or tap0 is also
   mirrored to gre0, a GRE tunnel to the remote host 192.168.1.10; any
   traffic arriving on gre0 is dropped:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0
       ovs-vsctl add-port br0 gre0 \
           -- set interface gre0 type=gre options:remote_ip=192.168.1.10 \
           -- --id=@p get port gre0 \
           -- --id=@m create mirror name=m0 select-all=true output-port=@p \
           -- set bridge br0 mirrors=@m

   To later disable mirroring and destroy the GRE tunnel:

       ovs-vsctl clear bridge br0 mirrors
       ovs-vcstl del-port br0 gre0

### Q: Does Open vSwitch support ERSPAN?

A: No.  ERSPAN is an undocumented proprietary protocol.  As an
   alternative, Open vSwitch supports mirroring to a GRE tunnel (see
   above).

### Q: How do I connect two bridges?

A: First, why do you want to do this?  Two connected bridges are not
   much different from a single bridge, so you might as well just have
   a single bridge with all your ports on it.

   If you still want to connect two bridges, you can use a pair of
   patch ports.  The following example creates bridges br0 and br1,
   adds eth0 and tap0 to br0, adds tap1 to br1, and then connects br0
   and br1 with a pair of patch ports.

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0
       ovs-vsctl add-br br1
       ovs-vsctl add-port br1 tap1
       ovs-vsctl \
           -- add-port br0 patch0 \
           -- set interface patch0 type=patch options:peer=patch1 \
           -- add-port br1 patch1 \
           -- set interface patch1 type=patch options:peer=patch0

   Bridges connected with patch ports are much like a single bridge.
   For instance, if the example above also added eth1 to br1, and both
   eth0 and eth1 happened to be connected to the same next-hop switch,
   then you could loop your network just as you would if you added
   eth0 and eth1 to the same bridge (see the "Configuration Problems"
   section below for more information).

   If you are using Open vSwitch 1.9 or an earlier version, then you
   need to be using the kernel module bundled with Open vSwitch rather
   than the one that is integrated into Linux 3.3 and later, because
   Open vSwitch 1.9 and earlier versions need kernel support for patch
   ports.  This also means that in Open vSwitch 1.9 and earlier, patch
   ports will not work with the userspace datapath, only with the
   kernel module.

### Q: How do I configure a bridge without an OpenFlow local port?  (Local port in the sense of OFPP_LOCAL)

A: Open vSwitch does not support such a configuration.
   Bridges always have their local ports.


Implementation Details
----------------------

### Q: I hear OVS has a couple of kinds of flows.  Can you tell me about them?

A: Open vSwitch uses different kinds of flows for different purposes:

  - OpenFlow flows are the most important kind of flow.  OpenFlow
    controllers use these flows to define a switch's policy.
    OpenFlow flows support wildcards, priorities, and multiple
    tables.

    When in-band control is in use, Open vSwitch sets up a few
    "hidden" flows, with priority higher than a controller or the
    user can configure, that are not visible via OpenFlow.  (See
    the "Controller" section of the FAQ for more information
    about hidden flows.)

  - The Open vSwitch software switch implementation uses a second
    kind of flow internally.  These flows, called "datapath" or
    "kernel" flows, do not support priorities and comprise only a
    single table, which makes them suitable for caching.  (Like
    OpenFlow flows, datapath flows do support wildcarding, in Open
    vSwitch 1.11 and later.)  OpenFlow flows and datapath flows
    also support different actions and number ports differently.

    Datapath flows are an implementation detail that is subject to
    change in future versions of Open vSwitch.  Even with the
    current version of Open vSwitch, hardware switch
    implementations do not necessarily use this architecture.

   Users and controllers directly control only the OpenFlow flow
   table.  Open vSwitch manages the datapath flow table itself, so
   users should not normally be concerned with it.

### Q: Why are there so many different ways to dump flows?

A: Open vSwitch has two kinds of flows (see the previous question), so
   it has commands with different purposes for dumping each kind of
   flow:

  - `ovs-ofctl dump-flows <br>` dumps OpenFlow flows, excluding
    hidden flows.  This is the most commonly useful form of flow
    dump.  (Unlike the other commands, this should work with any
    OpenFlow switch, not just Open vSwitch.)

  - `ovs-appctl bridge/dump-flows <br>` dumps OpenFlow flows,
    including hidden flows.  This is occasionally useful for
    troubleshooting suspected issues with in-band control.

  - `ovs-dpctl dump-flows [dp]` dumps the datapath flow table
    entries for a Linux kernel-based datapath.  In Open vSwitch
    1.10 and later, ovs-vswitchd merges multiple switches into a
    single datapath, so it will show all the flows on all your
    kernel-based switches.  This command can occasionally be
    useful for debugging.

  - `ovs-appctl dpif/dump-flows <br>`, new in Open vSwitch 1.10,
    dumps datapath flows for only the specified bridge, regardless
    of the type.

### Q: How does multicast snooping works with VLANs?

A: Open vSwitch maintains snooping tables for each VLAN.

### Q: Can OVS populate the kernel flow table in advance instead of in reaction to packets?

A: No.  There are several reasons:

  - Kernel flows are not as sophisticated as OpenFlow flows, which
    means that some OpenFlow policies could require a large number of
    kernel flows.  The "conjunctive match" feature is an extreme
    example: the number of kernel flows it requires is the product of
    the number of flows in each dimension.

  - With multiple OpenFlow flow tables and simple sets of actions, the
    number of kernel flows required can be as large as the product of
    the number of flows in each dimension.  With more sophisticated
    actions, the number of kernel flows could be even larger.

  - Open vSwitch is designed so that any version of OVS userspace
    interoperates with any version of the OVS kernel module.  This
    forward and backward compatibility requires that userspace observe
    how the kernel module parses received packets.  This is only
    possible in a straightforward way when userspace adds kernel flows
    in reaction to received packets.

  For more relevant information on the architecture of Open vSwitch,
  please read "The Design and Implementation of Open vSwitch",
  published in USENIX NSDI 2015.


Performance
-----------

### Q: I just upgraded and I see a performance drop.  Why?

A: The OVS kernel datapath may have been updated to a newer version than
   the OVS userspace components.  Sometimes new versions of OVS kernel
   module add functionality that is backwards compatible with older
   userspace components but may cause a drop in performance with them.
   Especially, if a kernel module from OVS 2.1 or newer is paired with
   OVS userspace 1.10 or older, there will be a performance drop for
   TCP traffic.

   Updating the OVS userspace components to the latest released
   version should fix the performance degradation.

   To get the best possible performance and functionality, it is
   recommended to pair the same versions of the kernel module and OVS
   userspace.


Configuration Problems
----------------------

### Q: I created a bridge and added my Ethernet port to it, using commands
   like these:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0

   and as soon as I ran the "add-port" command I lost all connectivity
   through eth0.  Help!

A: A physical Ethernet device that is part of an Open vSwitch bridge
   should not have an IP address.  If one does, then that IP address
   will not be fully functional.

   You can restore functionality by moving the IP address to an Open
   vSwitch "internal" device, such as the network device named after
   the bridge itself.  For example, assuming that eth0's IP address is
   192.168.128.5, you could run the commands below to fix up the
   situation:

       ifconfig eth0 0.0.0.0
       ifconfig br0 192.168.128.5

   (If your only connection to the machine running OVS is through the
   IP address in question, then you would want to run all of these
   commands on a single command line, or put them into a script.)  If
   there were any additional routes assigned to eth0, then you would
   also want to use commands to adjust these routes to go through br0.

   If you use DHCP to obtain an IP address, then you should kill the
   DHCP client that was listening on the physical Ethernet interface
   (e.g. eth0) and start one listening on the internal interface
   (e.g. br0).  You might still need to manually clear the IP address
   from the physical interface (e.g. with "ifconfig eth0 0.0.0.0").

   There is no compelling reason why Open vSwitch must work this way.
   However, this is the way that the Linux kernel bridge module has
   always worked, so it's a model that those accustomed to Linux
   bridging are already used to.  Also, the model that most people
   expect is not implementable without kernel changes on all the
   versions of Linux that Open vSwitch supports.

   By the way, this issue is not specific to physical Ethernet
   devices.  It applies to all network devices except Open vSwitch
   "internal" devices.

### Q: I created a bridge and added a couple of Ethernet ports to it,
### using commands like these:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 eth1

   and now my network seems to have melted: connectivity is unreliable
   (even connectivity that doesn't go through Open vSwitch), all the
   LEDs on my physical switches are blinking, wireshark shows
   duplicated packets, and CPU usage is very high.

A: More than likely, you've looped your network.  Probably, eth0 and
   eth1 are connected to the same physical Ethernet switch.  This
   yields a scenario where OVS receives a broadcast packet on eth0 and
   sends it out on eth1, then the physical switch connected to eth1
   sends the packet back on eth0, and so on forever.  More complicated
   scenarios, involving a loop through multiple switches, are possible
   too.

   The solution depends on what you are trying to do:

   - If you added eth0 and eth1 to get higher bandwidth or higher
     reliability between OVS and your physical Ethernet switch,
     use a bond.  The following commands create br0 and then add
     eth0 and eth1 as a bond:

         ovs-vsctl add-br br0
         ovs-vsctl add-bond br0 bond0 eth0 eth1

     Bonds have tons of configuration options.  Please read the
     documentation on the Port table in ovs-vswitchd.conf.db(5)
     for all the details.

     Configuration for DPDK-enabled interfaces is slightly less
     straightforward: see [INSTALL.DPDK.md].

   - Perhaps you don't actually need eth0 and eth1 to be on the
     same bridge.  For example, if you simply want to be able to
     connect each of them to virtual machines, then you can put
     each of them on a bridge of its own:

         ovs-vsctl add-br br0
         ovs-vsctl add-port br0 eth0

         ovs-vsctl add-br br1
         ovs-vsctl add-port br1 eth1

     and then connect VMs to br0 and br1.  (A potential
     disadvantage is that traffic cannot directly pass between br0
     and br1.  Instead, it will go out eth0 and come back in eth1,
     or vice versa.)

   - If you have a redundant or complex network topology and you
     want to prevent loops, turn on spanning tree protocol (STP).
     The following commands create br0, enable STP, and add eth0
     and eth1 to the bridge.  The order is important because you
     don't want have to have a loop in your network even
     transiently:

         ovs-vsctl add-br br0
         ovs-vsctl set bridge br0 stp_enable=true
         ovs-vsctl add-port br0 eth0
         ovs-vsctl add-port br0 eth1

     The Open vSwitch implementation of STP is not well tested.
     Please report any bugs you observe, but if you'd rather avoid
     acting as a beta tester then another option might be your
     best shot.

### Q: I can't seem to use Open vSwitch in a wireless network.

A: Wireless base stations generally only allow packets with the source
   MAC address of NIC that completed the initial handshake.
   Therefore, without MAC rewriting, only a single device can
   communicate over a single wireless link.

   This isn't specific to Open vSwitch, it's enforced by the access
   point, so the same problems will show up with the Linux bridge or
   any other way to do bridging.

### Q: I can't seem to add my PPP interface to an Open vSwitch bridge.

A: PPP most commonly carries IP packets, but Open vSwitch works only
   with Ethernet frames.  The correct way to interface PPP to an
   Ethernet network is usually to use routing instead of switching.

### Q: Is there any documentation on the database tables and fields?

A: Yes.  ovs-vswitchd.conf.db(5) is a comprehensive reference.

### Q: When I run ovs-dpctl I no longer see the bridges I created.  Instead,
   I only see a datapath called "ovs-system".  How can I see datapath
   information about a particular bridge?

A: In version 1.9.0, OVS switched to using a single datapath that is
   shared by all bridges of that type.  The "ovs-appctl dpif/*"
   commands provide similar functionality that is scoped by the bridge.

### Q: I created a GRE port using ovs-vsctl so why can't I send traffic or
   see the port in the datapath?

A: On Linux kernels before 3.11, the OVS GRE module and Linux GRE module
   cannot be loaded at the same time. It is likely that on your system the
   Linux GRE module is already loaded and blocking OVS (to confirm, check
   dmesg for errors regarding GRE registration). To fix this, unload all
   GRE modules that appear in lsmod as well as the OVS kernel module. You
   can then reload the OVS module following the directions in
   [INSTALL.md], which will ensure that dependencies are satisfied.

### Q: Open vSwitch does not seem to obey my packet filter rules.

A: It depends on mechanisms and configurations you want to use.

   You cannot usefully use typical packet filters, like iptables, on
   physical Ethernet ports that you add to an Open vSwitch bridge.
   This is because Open vSwitch captures packets from the interface at
   a layer lower below where typical packet-filter implementations
   install their hooks.  (This actually applies to any interface of
   type "system" that you might add to an Open vSwitch bridge.)

   You can usefully use typical packet filters on Open vSwitch
   internal ports as they are mostly ordinary interfaces from the point
   of view of packet filters.

   For example, suppose you create a bridge br0 and add Ethernet port
   eth0 to it.  Then you can usefully add iptables rules to affect the
   internal interface br0, but not the physical interface eth0.  (br0
   is also where you would add an IP address, as discussed elsewhere
   in the FAQ.)

   For simple filtering rules, it might be possible to achieve similar
   results by installing appropriate OpenFlow flows instead.

   If the use of a particular packet filter setup is essential, Open
   vSwitch might not be the best choice for you.  On Linux, you might
   want to consider using the Linux Bridge.  (This is the only choice if
   you want to use ebtables rules.)  On NetBSD, you might want to
   consider using the bridge(4) with BRIDGE_IPF option.

### Q: It seems that Open vSwitch does nothing when I removed a port and
   then immediately put it back.  For example, consider that p1 is
   a port of type=internal:

       ovs-vsctl del-port br0 p1 -- \
           add-port br0 p1 -- \
           set interface p1 type=internal

A: It's an expected behaviour.

   If del-port and add-port happen in a single OVSDB transaction as
   your example, Open vSwitch always "skips" the intermediate steps.
   Even if they are done in multiple transactions, it's still allowed
   for Open vSwitch to skip the intermediate steps and just implement
   the overall effect.  In both cases, your example would be turned
   into a no-op.

   If you want to make Open vSwitch actually destroy and then re-create
   the port for some side effects like resetting kernel setting for the
   corresponding interface, you need to separate operations into multiple
   OVSDB transactions and ensure that at least the first one does not have
   --no-wait.  In the following example, the first ovs-vsctl will block
   until Open vSwitch reloads the new configuration and removes the port:

       ovs-vsctl del-port br0 p1
       ovs-vsctl add-port br0 p1 -- \
           set interface p1 type=internal

### Q: I want to add thousands of ports to an Open vSwitch bridge, but
   it takes too long (minutes or hours) to do it with ovs-vsctl.  How
   can I do it faster?

A: If you add them one at a time with ovs-vsctl, it can take a long
   time to add thousands of ports to an Open vSwitch bridge.  This is
   because every invocation of ovs-vsctl first reads the current
   configuration from OVSDB.  As the number of ports grows, this
   starts to take an appreciable amount of time, and when it is
   repeated thousands of times the total time becomes significant.

   The solution is to add the ports in one invocation of ovs-vsctl (or
   a small number of them).  For example, using bash:

       ovs-vsctl add-br br0
       cmds=; for i in {1..5000}; do cmds+=" -- add-port br0 p$i"; done
       ovs-vsctl $cmds

   takes seconds, not minutes or hours, in the OVS sandbox environment.

### Q: I created a bridge named br0.  My bridge shows up in "ovs-vsctl
    show", but "ovs-ofctl show br0" just prints "br0 is not a bridge
    or a socket".

A: Open vSwitch wasn't able to create the bridge.  Check the
   ovs-vswitchd log for details (Debian and Red Hat packaging for Open
   vSwitch put it in /var/log/openvswitch/ovs-vswitchd.log).

   In general, the Open vSwitch database reflects the desired
   configuration state.  ovs-vswitchd monitors the database and, when
   it changes, reconfigures the system to reflect the new desired
   state.  This normally happens very quickly.  Thus, a discrepancy
   between the database and the actual state indicates that
   ovs-vswitchd could not implement the configuration, and so one
   should check the log to find out why.  (Another possible cause is
   that ovs-vswitchd is not running.  This will make "ovs-vsctl"
   commands hang, if they change the configuration, unless one
   specifies "--no-wait".)

### Q: I have a bridge br0.  I added a new port vif1.0, and it shows
    up in "ovs-vsctl show", but "ovs-vsctl list port" says that it has
    OpenFlow port ("ofport") -1, and "ovs-ofctl show br0" doesn't show
    vif1.0 at all.

A: Open vSwitch wasn't able to create the port.  Check the
   ovs-vswitchd log for details (Debian and Red Hat packaging for Open
   vSwitch put it in /var/log/openvswitch/ovs-vswitchd.log).  Please
   see the previous question for more information.

   You may want to upgrade to Open vSwitch 2.3 (or later), in which
   ovs-vsctl will immediately report when there is an issue creating a
   port.

### Q: I created a tap device tap0, configured an IP address on it, and
    added it to a bridge, like this:

        tunctl -t tap0
        ifconfig tap0 192.168.0.123
        ovs-vsctl add-br br0
        ovs-vsctl add-port br0 tap0

    I expected that I could then use this IP address to contact other
    hosts on the network, but it doesn't work.  Why not?

A: The short answer is that this is a misuse of a "tap" device.  Use
   an "internal" device implemented by Open vSwitch, which works
   differently and is designed for this use.  To solve this problem
   with an internal device, instead run:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 int0 -- set Interface int0 type=internal
       ifconfig int0 192.168.0.123

   Even more simply, you can take advantage of the internal port that
   every bridge has under the name of the bridge:

       ovs-vsctl add-br br0
       ifconfig br0 192.168.0.123

   In more detail, a "tap" device is an interface between the Linux
   (or *BSD) network stack and a user program that opens it as a
   socket.  When the "tap" device transmits a packet, it appears in
   the socket opened by the userspace program.  Conversely, when the
   userspace program writes to the "tap" socket, the kernel TCP/IP
   stack processes the packet as if it had been received by the "tap"
   device.

   Consider the configuration above.  Given this configuration, if you
   "ping" an IP address in the 192.168.0.x subnet, the Linux kernel
   routing stack will transmit an ARP on the tap0 device.  Open
   vSwitch userspace treats "tap" devices just like any other network
   device; that is, it doesn't open them as "tap" sockets.  That means
   that the ARP packet will simply get dropped.

   You might wonder why the Open vSwitch kernel module doesn't
   intercept the ARP packet and bridge it.  After all, Open vSwitch
   intercepts packets on other devices.  The answer is that Open
   vSwitch only intercepts *received* packets, but this is a packet
   being transmitted.  The same thing happens for all other types of
   network devices, except for Open vSwitch "internal" ports.  If you,
   for example, add a physical Ethernet port to an OVS bridge,
   configure an IP address on a physical Ethernet port, and then issue
   a "ping" to an address in that subnet, the same thing happens: an
   ARP gets transmitted on the physical Ethernet port and Open vSwitch
   never sees it.  (You should not do that, as documented at the
   beginning of this section.)

   It can make sense to add a "tap" device to an Open vSwitch bridge,
   if some userspace program (other than Open vSwitch) has opened the
   tap socket.  This is the case, for example, if the "tap" device was
   created by KVM (or QEMU) to simulate a virtual NIC.  In such a
   case, when OVS bridges a packet to the "tap" device, the kernel
   forwards that packet to KVM in userspace, which passes it along to
   the VM, and in the other direction, when the VM sends a packet, KVM
   writes it to the "tap" socket, which causes OVS to receive it and
   bridge it to the other OVS ports.  Please note that in such a case
   no IP address is configured on the "tap" device (there is normally
   an IP address configured in the virtual NIC inside the VM, but this
   is not visible to the host Linux kernel or to Open vSwitch).

   There is one special case in which Open vSwitch does directly read
   and write "tap" sockets.  This is an implementation detail of the
   Open vSwitch userspace switch, which implements its "internal"
   ports as Linux (or *BSD) "tap" sockets.  In such a userspace
   switch, OVS receives packets sent on the "tap" device used to
   implement an "internal" port by reading the associated "tap"
   socket, and bridges them to the rest of the switch.  In the other
   direction, OVS transmits packets bridged to the "internal" port by
   writing them to the "tap" socket, causing them to be processed by
   the kernel TCP/IP stack as if they had been received on the "tap"
   device.  Users should not need to be concerned with this
   implementation detail.

   Open vSwitch has a network device type called "tap".  This is
   intended only for implementing "internal" ports in the OVS
   userspace switch and should not be used otherwise.  In particular,
   users should not configure KVM "tap" devices as type "tap" (use
   type "system", the default, instead).


Quality of Service (QoS)
------------------------

### Q: Does OVS support Quality of Service (QoS)?

A: Yes.  For traffic that egresses from a switch, OVS supports traffic
   shaping; for traffic that ingresses into a switch, OVS support
   policing.  Policing is a simple form of quality-of-service that
   simply drops packets received in excess of the configured rate.  Due
   to its simplicity, policing is usually less accurate and less
   effective than egress traffic shaping, which queues packets.

   Keep in mind that ingress and egress are from the perspective of the
   switch.  That means that egress shaping limits the rate at which
   traffic is allowed to transmit from a physical interface, but the
   rate at which traffic will be received on a virtual machine's VIF.
   For ingress policing, the behavior is the opposite.

### Q: How do I configure egress traffic shaping?

A: Suppose that you want to set up bridge br0 connected to physical
   Ethernet port eth0 (a 1 Gbps device) and virtual machine interfaces
   vif1.0 and vif2.0, and that you want to limit traffic from vif1.0
   to eth0 to 10 Mbps and from vif2.0 to eth0 to 20 Mbps.  Then, you
   could configure the bridge this way:

       ovs-vsctl -- \
           add-br br0 -- \
           add-port br0 eth0 -- \
           add-port br0 vif1.0 -- set interface vif1.0 ofport_request=5 -- \
           add-port br0 vif2.0 -- set interface vif2.0 ofport_request=6 -- \
           set port eth0 qos=@newqos -- \
           --id=@newqos create qos type=linux-htb \
               other-config:max-rate=1000000000 \
               queues:123=@vif10queue \
               queues:234=@vif20queue -- \
           --id=@vif10queue create queue other-config:max-rate=10000000 -- \
           --id=@vif20queue create queue other-config:max-rate=20000000

   At this point, bridge br0 is configured with the ports and eth0 is
   configured with the queues that you need for QoS, but nothing is
   actually directing packets from vif1.0 or vif2.0 to the queues that
   we have set up for them.  That means that all of the packets to
   eth0 are going to the "default queue", which is not what we want.

   We use OpenFlow to direct packets from vif1.0 and vif2.0 to the
   queues reserved for them:

       ovs-ofctl add-flow br0 in_port=5,actions=set_queue:123,normal
       ovs-ofctl add-flow br0 in_port=6,actions=set_queue:234,normal

   Each of the above flows matches on the input port, sets up the
   appropriate queue (123 for vif1.0, 234 for vif2.0), and then
   executes the "normal" action, which performs the same switching
   that Open vSwitch would have done without any OpenFlow flows being
   present.  (We know that vif1.0 and vif2.0 have OpenFlow port
   numbers 5 and 6, respectively, because we set their ofport_request
   columns above.  If we had not done that, then we would have needed
   to find out their port numbers before setting up these flows.)

   Now traffic going from vif1.0 or vif2.0 to eth0 should be
   rate-limited.

   By the way, if you delete the bridge created by the above commands,
   with:

       ovs-vsctl del-br br0

   then that will leave one unreferenced QoS record and two
   unreferenced Queue records in the Open vSwich database.  One way to
   clear them out, assuming you don't have other QoS or Queue records
   that you want to keep, is:

       ovs-vsctl -- --all destroy QoS -- --all destroy Queue

   If you do want to keep some QoS or Queue records, or the Open
   vSwitch you are using is older than version 1.8 (which added the
   --all option), then you will have to destroy QoS and Queue records
   individually.

### Q: How do I configure ingress policing?

A: A policing policy can be configured on an interface to drop packets
   that arrive at a higher rate than the configured value.  For example,
   the following commands will rate-limit traffic that vif1.0 may
   generate to 10Mbps:

       ovs-vsctl set interface vif1.0 ingress_policing_rate=10000
       ovs-vsctl set interface vif1.0 ingress_policing_burst=1000

   Traffic policing can interact poorly with some network protocols and
   can have surprising results.  The "Ingress Policing" section of
   ovs-vswitchd.conf.db(5) discusses the issues in greater detail.

### Q: I configured Quality of Service (QoS) in my OpenFlow network by
   adding records to the QoS and Queue table, but the results aren't
   what I expect.

A: Did you install OpenFlow flows that use your queues?  This is the
   primary way to tell Open vSwitch which queues you want to use.  If
   you don't do this, then the default queue will be used, which will
   probably not have the effect you want.

   Refer to the previous question for an example.

### Q: I'd like to take advantage of some QoS feature that Open vSwitch
   doesn't yet support.  How do I do that?

A: Open vSwitch does not implement QoS itself.  Instead, it can
   configure some, but not all, of the QoS features built into the
   Linux kernel.  If you need some QoS feature that OVS cannot
   configure itself, then the first step is to figure out whether
   Linux QoS supports that feature.  If it does, then you can submit a
   patch to support Open vSwitch configuration for that feature, or
   you can use "tc" directly to configure the feature in Linux.  (If
   Linux QoS doesn't support the feature you want, then first you have
   to add that support to Linux.)

### Q: I configured QoS, correctly, but my measurements show that it isn't
   working as well as I expect.

A: With the Linux kernel, the Open vSwitch implementation of QoS has
   two aspects:

   - Open vSwitch configures a subset of Linux kernel QoS
     features, according to what is in OVSDB.  It is possible that
     this code has bugs.  If you believe that this is so, then you
     can configure the Linux traffic control (QoS) stack directly
     with the "tc" program.  If you get better results that way,
     you can send a detailed bug report to bugs@openvswitch.org.

     It is certain that Open vSwitch cannot configure every Linux
     kernel QoS feature.  If you need some feature that OVS cannot
     configure, then you can also use "tc" directly (or add that
     feature to OVS).

   - The Open vSwitch implementation of OpenFlow allows flows to
     be directed to particular queues.  This is pretty simple and
     unlikely to have serious bugs at this point.

   However, most problems with QoS on Linux are not bugs in Open
   vSwitch at all.  They tend to be either configuration errors
   (please see the earlier questions in this section) or issues with
   the traffic control (QoS) stack in Linux.  The Open vSwitch
   developers are not experts on Linux traffic control.  We suggest
   that, if you believe you are encountering a problem with Linux
   traffic control, that you consult the tc manpages (e.g. tc(8),
   tc-htb(8), tc-hfsc(8)), web resources (e.g. http://lartc.org/), or
   mailing lists (e.g. http://vger.kernel.org/vger-lists.html#netdev).

### Q: Does Open vSwitch support OpenFlow meters?

A: Since version 2.0, Open vSwitch has OpenFlow protocol support for
   OpenFlow meters.  There is no implementation of meters in the Open
   vSwitch software switch (neither the kernel-based nor userspace
   switches).


VLANs
-----

### Q: What's a VLAN?

A: At the simplest level, a VLAN (short for "virtual LAN") is a way to
   partition a single switch into multiple switches.  Suppose, for
   example, that you have two groups of machines, group A and group B.
   You want the machines in group A to be able to talk to each other,
   and you want the machine in group B to be able to talk to each
   other, but you don't want the machines in group A to be able to
   talk to the machines in group B.  You can do this with two
   switches, by plugging the machines in group A into one switch and
   the machines in group B into the other switch.

   If you only have one switch, then you can use VLANs to do the same
   thing, by configuring the ports for machines in group A as VLAN
   "access ports" for one VLAN and the ports for group B as "access
   ports" for a different VLAN.  The switch will only forward packets
   between ports that are assigned to the same VLAN, so this
   effectively subdivides your single switch into two independent
   switches, one for each group of machines.

   So far we haven't said anything about VLAN headers.  With access
   ports, like we've described so far, no VLAN header is present in
   the Ethernet frame.  This means that the machines (or switches)
   connected to access ports need not be aware that VLANs are
   involved, just like in the case where we use two different physical
   switches.

   Now suppose that you have a whole bunch of switches in your
   network, instead of just one, and that some machines in group A are
   connected directly to both switches 1 and 2.  To allow these
   machines to talk to each other, you could add an access port for
   group A's VLAN to switch 1 and another to switch 2, and then
   connect an Ethernet cable between those ports.  That works fine,
   but it doesn't scale well as the number of switches and the number
   of VLANs increases, because you use up a lot of valuable switch
   ports just connecting together your VLANs.

   This is where VLAN headers come in.  Instead of using one cable and
   two ports per VLAN to connect a pair of switches, we configure a
   port on each switch as a VLAN "trunk port".  Packets sent and
   received on a trunk port carry a VLAN header that says what VLAN
   the packet belongs to, so that only two ports total are required to
   connect the switches, regardless of the number of VLANs in use.
   Normally, only switches (either physical or virtual) are connected
   to a trunk port, not individual hosts, because individual hosts
   don't expect to see a VLAN header in the traffic that they receive.

   None of the above discussion says anything about particular VLAN
   numbers.  This is because VLAN numbers are completely arbitrary.
   One must only ensure that a given VLAN is numbered consistently
   throughout a network and that different VLANs are given different
   numbers.  (That said, VLAN 0 is usually synonymous with a packet
   that has no VLAN header, and VLAN 4095 is reserved.)

### Q: VLANs don't work.

A: Many drivers in Linux kernels before version 3.3 had VLAN-related
   bugs.  If you are having problems with VLANs that you suspect to be
   driver related, then you have several options:

   - Upgrade to Linux 3.3 or later.

   - Build and install a fixed version of the particular driver
     that is causing trouble, if one is available.

   - Use a NIC whose driver does not have VLAN problems.

   - Use "VLAN splinters", a feature in Open vSwitch 1.4 and later
     that works around bugs in kernel drivers.  To enable VLAN
     splinters on interface eth0, use the command:

       ovs-vsctl set interface eth0 other-config:enable-vlan-splinters=true

     For VLAN splinters to be effective, Open vSwitch must know
     which VLANs are in use.  See the "VLAN splinters" section in
     the Interface table in ovs-vswitchd.conf.db(5) for details on
     how Open vSwitch infers in-use VLANs.

     VLAN splinters increase memory use and reduce performance, so
     use them only if needed.

   - Apply the "vlan workaround" patch from the XenServer kernel
     patch queue, build Open vSwitch against this patched kernel,
     and then use ovs-vlan-bug-workaround(8) to enable the VLAN
     workaround for each interface whose driver is buggy.

     (This is a nontrivial exercise, so this option is included
     only for completeness.)

   It is not always easy to tell whether a Linux kernel driver has
   buggy VLAN support.  The ovs-vlan-test(8) and ovs-test(8) utilities
   can help you test.  See their manpages for details.  Of the two
   utilities, ovs-test(8) is newer and more thorough, but
   ovs-vlan-test(8) may be easier to use.

### Q: VLANs still don't work.  I've tested the driver so I know that it's OK.

A: Do you have VLANs enabled on the physical switch that OVS is
   attached to?  Make sure that the port is configured to trunk the
   VLAN or VLANs that you are using with OVS.

### Q: Outgoing VLAN-tagged traffic goes through OVS to my physical switch
   and to its destination host, but OVS seems to drop incoming return
   traffic.

A: It's possible that you have the VLAN configured on your physical
   switch as the "native" VLAN.  In this mode, the switch treats
   incoming packets either tagged with the native VLAN or untagged as
   part of the native VLAN.  It may also send outgoing packets in the
   native VLAN without a VLAN tag.

   If this is the case, you have two choices:

   - Change the physical switch port configuration to tag packets
     it forwards to OVS with the native VLAN instead of forwarding
     them untagged.

   - Change the OVS configuration for the physical port to a
     native VLAN mode.  For example, the following sets up a
     bridge with port eth0 in "native-tagged" mode in VLAN 9:

         ovs-vsctl add-br br0
         ovs-vsctl add-port br0 eth0 tag=9 vlan_mode=native-tagged

     In this situation, "native-untagged" mode will probably work
     equally well.  Refer to the documentation for the Port table
     in ovs-vswitchd.conf.db(5) for more information.

### Q: I added a pair of VMs on different VLANs, like this:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0 tag=9
       ovs-vsctl add-port br0 tap1 tag=10

    but the VMs can't access each other, the external network, or the
    Internet.

A: It is to be expected that the VMs can't access each other.  VLANs
   are a means to partition a network.  When you configured tap0 and
   tap1 as access ports for different VLANs, you indicated that they
   should be isolated from each other.

   As for the external network and the Internet, it seems likely that
   the machines you are trying to access are not on VLAN 9 (or 10) and
   that the Internet is not available on VLAN 9 (or 10).

### Q: I added a pair of VMs on the same VLAN, like this:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0 tag=9
       ovs-vsctl add-port br0 tap1 tag=9

    The VMs can access each other, but not the external network or the
    Internet.

A: It seems likely that the machines you are trying to access in the
   external network are not on VLAN 9 and that the Internet is not
   available on VLAN 9.  Also, ensure VLAN 9 is set up as an allowed
   trunk VLAN on the upstream switch port to which eth0 is connected.

### Q: Can I configure an IP address on a VLAN?

A: Yes.  Use an "internal port" configured as an access port.  For
   example, the following configures IP address 192.168.0.7 on VLAN 9.
   That is, OVS will forward packets from eth0 to 192.168.0.7 only if
   they have an 802.1Q header with VLAN 9.  Conversely, traffic
   forwarded from 192.168.0.7 to eth0 will be tagged with an 802.1Q
   header with VLAN 9:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 vlan9 tag=9 -- set interface vlan9 type=internal
       ifconfig vlan9 192.168.0.7

   See also the following question.

### Q: I configured one IP address on VLAN 0 and another on VLAN 9, like
   this:

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 eth0
       ifconfig br0 192.168.0.5
       ovs-vsctl add-port br0 vlan9 tag=9 -- set interface vlan9 type=internal
       ifconfig vlan9 192.168.0.9

   but other hosts that are only on VLAN 0 can reach the IP address
   configured on VLAN 9.  What's going on?

A: RFC 1122 section 3.3.4.2 "Multihoming Requirements" describes two
   approaches to IP address handling in Internet hosts:

   - In the "Strong ES Model", where an ES is a host ("End
     System"), an IP address is primarily associated with a
     particular interface.  The host discards packets that arrive
     on interface A if they are destined for an IP address that is
     configured on interface B.  The host never sends packets from
     interface A using a source address configured on interface B.

   - In the "Weak ES Model", an IP address is primarily associated
     with a host.  The host accepts packets that arrive on any
     interface if they are destined for any of the host's IP
     addresses, even if the address is configured on some
     interface other than the one on which it arrived.  The host
     does not restrict itself to sending packets from an IP
     address associated with the originating interface.

   Linux uses the weak ES model.  That means that when packets
   destined to the VLAN 9 IP address arrive on eth0 and are bridged to
   br0, the kernel IP stack accepts them there for the VLAN 9 IP
   address, even though they were not received on vlan9, the network
   device for vlan9.

   To simulate the strong ES model on Linux, one may add iptables rule
   to filter packets based on source and destination address and
   adjust ARP configuration with sysctls.

   BSD uses the strong ES model.

### Q: My OpenFlow controller doesn't see the VLANs that I expect.

A: The configuration for VLANs in the Open vSwitch database (e.g. via
   ovs-vsctl) only affects traffic that goes through Open vSwitch's
   implementation of the OpenFlow "normal switching" action.  By
   default, when Open vSwitch isn't connected to a controller and
   nothing has been manually configured in the flow table, all traffic
   goes through the "normal switching" action.  But, if you set up
   OpenFlow flows on your own, through a controller or using ovs-ofctl
   or through other means, then you have to implement VLAN handling
   yourself.

   You can use "normal switching" as a component of your OpenFlow
   actions, e.g. by putting "normal" into the lists of actions on
   ovs-ofctl or by outputting to OFPP_NORMAL from an OpenFlow
   controller.  In situations where this is not suitable, you can
   implement VLAN handling yourself, e.g.:

   - If a packet comes in on an access port, and the flow table
     needs to send it out on a trunk port, then the flow can add
     the appropriate VLAN tag with the "mod_vlan_vid" action.

   - If a packet comes in on a trunk port, and the flow table
     needs to send it out on an access port, then the flow can
     strip the VLAN tag with the "strip_vlan" action.

### Q: I configured ports on a bridge as access ports with different VLAN
   tags, like this:

       ovs-vsctl add-br br0
       ovs-vsctl set-controller br0 tcp:192.168.0.10:6653
       ovs-vsctl add-port br0 eth0
       ovs-vsctl add-port br0 tap0 tag=9
       ovs-vsctl add-port br0 tap1 tag=10

   but the VMs running behind tap0 and tap1 can still communicate,
   that is, they are not isolated from each other even though they are
   on different VLANs.

A: Do you have a controller configured on br0 (as the commands above
   do)?  If so, then this is a variant on the previous question, "My
   OpenFlow controller doesn't see the VLANs that I expect," and you
   can refer to the answer there for more information.

### Q: How MAC learning works with VLANs?

A: Open vSwitch implements Independent VLAN Learning (IVL) for
   OFPP_NORMAL action.  I.e. it logically has separate learning tables
   for each VLANs.


VXLANs
-----

### Q: What's a VXLAN?

A: VXLAN stands for Virtual eXtensible Local Area Network, and is a means
   to solve the scaling challenges of VLAN networks in a multi-tenant
   environment. VXLAN is an overlay network which transports an L2 network
   over an existing L3 network. For more information on VXLAN, please see
   RFC 7348:

   http://tools.ietf.org/html/rfc7348

### Q: How much of the VXLAN protocol does Open vSwitch currently support?

A: Open vSwitch currently supports the framing format for packets on the
   wire. There is currently no support for the multicast aspects of VXLAN.
   To get around the lack of multicast support, it is possible to
   pre-provision MAC to IP address mappings either manually or from a
   controller.

### Q: What destination UDP port does the VXLAN implementation in Open vSwitch
   use?

A: By default, Open vSwitch will use the assigned IANA port for VXLAN, which
   is 4789. However, it is possible to configure the destination UDP port
   manually on a per-VXLAN tunnel basis. An example of this configuration is
   provided below.

       ovs-vsctl add-br br0
       ovs-vsctl add-port br0 vxlan1 -- set interface vxlan1
       type=vxlan options:remote_ip=192.168.1.2 options:key=flow
       options:dst_port=8472


Using OpenFlow (Manually or Via Controller)
-------------------------------------------

### Q: What versions of OpenFlow does Open vSwitch support?

A: The following table lists the versions of OpenFlow supported by
   each version of Open vSwitch:

       Open vSwitch      OF1.0  OF1.1  OF1.2  OF1.3  OF1.4  OF1.5
       ###============   =====  =====  =====  =====  =====  =====
       1.9 and earlier    yes    ---    ---    ---    ---    ---
       1.10               yes    ---    [*]    [*]    ---    ---
       1.11               yes    ---    [*]    [*]    ---    ---
       2.0                yes    [*]    [*]    [*]    ---    ---
       2.1                yes    [*]    [*]    [*]    ---    ---
       2.2                yes    [*]    [*]    [*]    [%]    [*]
       2.3                yes    yes    yes    yes    [*]    [*]

       [*] Supported, with one or more missing features.
       [%] Experimental, unsafe implementation.

   Open vSwitch 2.3 enables OpenFlow 1.0, 1.1, 1.2, and 1.3 by default
   in ovs-vswitchd.  In Open vSwitch 1.10 through 2.2, OpenFlow 1.1,
   1.2, and 1.3 must be enabled manually in ovs-vswitchd.  OpenFlow
   1.4 and 1.5 are also supported, with missing features, in Open
   vSwitch 2.3 and later, but not enabled by default.  In any case,
   the user may override the default:

   - To enable OpenFlow 1.0, 1.1, 1.2, and 1.3 on bridge br0:

     ovs-vsctl set bridge br0 protocols=OpenFlow10,OpenFlow11,OpenFlow12,OpenFlow13

   - To enable OpenFlow 1.0, 1.1, 1.2, 1.3, 1.4, and 1.5 on bridge br0:

     ovs-vsctl set bridge br0 protocols=OpenFlow10,OpenFlow11,OpenFlow12,OpenFlow13,OpenFlow14,OpenFlow15

   - To enable only OpenFlow 1.0 on bridge br0:

     ovs-vsctl set bridge br0 protocols=OpenFlow10

   All current versions of ovs-ofctl enable only OpenFlow 1.0 by
   default.  Use the -O option to enable support for later versions of
   OpenFlow in ovs-ofctl.  For example:

       ovs-ofctl -O OpenFlow13 dump-flows br0

   (Open vSwitch 2.2 had an experimental implementation of OpenFlow
   1.4 that could cause crashes.  We don't recommend enabling it.)

   [OPENFLOW-1.1+.md] in the Open vSwitch source tree tracks support for
   OpenFlow 1.1 and later features.  When support for OpenFlow 1.4 and
   1.5 is solidly implemented, Open vSwitch will enable those version
   by default.

### Q: Does Open vSwitch support MPLS?

A: Before version 1.11, Open vSwitch did not support MPLS.  That is,
   these versions can match on MPLS Ethernet types, but they cannot
   match, push, or pop MPLS labels, nor can they look past MPLS labels
   into the encapsulated packet.

   Open vSwitch versions 1.11, 2.0, and 2.1 have very minimal support
   for MPLS.  With the userspace datapath only, these versions can
   match, push, or pop a single MPLS label, but they still cannot look
   past MPLS labels (even after popping them) into the encapsulated
   packet.  Kernel datapath support is unchanged from earlier
   versions.

   Open vSwitch version 2.3 can match, push, or pop a single MPLS
   label and look past the MPLS label into the encapsulated packet.
   Both userspace and kernel datapaths will be supported, but MPLS
   processing always happens in userspace either way, so kernel
   datapath performance will be disappointing.

   Open vSwitch version 2.4 can match, push, or pop up to 3 MPLS
   labels and look past the MPLS label into the encapsulated packet.
   It will have kernel support for MPLS, yielding improved
   performance.

### Q: I'm getting "error type 45250 code 0".  What's that?

A: This is a Open vSwitch extension to OpenFlow error codes.  Open
   vSwitch uses this extension when it must report an error to an
   OpenFlow controller but no standard OpenFlow error code is
   suitable.

   Open vSwitch logs the errors that it sends to controllers, so the
   easiest thing to do is probably to look at the ovs-vswitchd log to
   find out what the error was.

   If you want to dissect the extended error message yourself, the
   format is documented in include/openflow/nicira-ext.h in the Open
   vSwitch source distribution.  The extended error codes are
   documented in lib/ofp-errors.h.

Q1: Some of the traffic that I'd expect my OpenFlow controller to see
    doesn't actually appear through the OpenFlow connection, even
    though I know that it's going through.
Q2: Some of the OpenFlow flows that my controller sets up don't seem
    to apply to certain traffic, especially traffic between OVS and
    the controller itself.

A: By default, Open vSwitch assumes that OpenFlow controllers are
   connected "in-band", that is, that the controllers are actually
   part of the network that is being controlled.  In in-band mode,
   Open vSwitch sets up special "hidden" flows to make sure that
   traffic can make it back and forth between OVS and the controllers.
   These hidden flows are higher priority than any flows that can be
   set up through OpenFlow, and they are not visible through normal
   OpenFlow flow table dumps.

   Usually, the hidden flows are desirable and helpful, but
   occasionally they can cause unexpected behavior.  You can view the
   full OpenFlow flow table, including hidden flows, on bridge br0
   with the command:

       ovs-appctl bridge/dump-flows br0

   to help you debug.  The hidden flows are those with priorities
   greater than 65535 (the maximum priority that can be set with
   OpenFlow).

   The DESIGN file at the top level of the Open vSwitch source
   distribution describes the in-band model in detail.

   If your controllers are not actually in-band (e.g. they are on
   localhost via 127.0.0.1, or on a separate network), then you should
   configure your controllers in "out-of-band" mode.  If you have one
   controller on bridge br0, then you can configure out-of-band mode
   on it with:

       ovs-vsctl set controller br0 connection-mode=out-of-band

### Q: I configured all my controllers for out-of-band control mode but
   "ovs-appctl bridge/dump-flows" still shows some hidden flows.

A: You probably have a remote manager configured (e.g. with "ovs-vsctl
   set-manager").  By default, Open vSwitch assumes that managers need
   in-band rules set up on every bridge.  You can disable these rules
   on bridge br0 with:

       ovs-vsctl set bridge br0 other-config:disable-in-band=true

   This actually disables in-band control entirely for the bridge, as
   if all the bridge's controllers were configured for out-of-band
   control.

### Q: My OpenFlow controller doesn't see the VLANs that I expect.

A: See answer under "VLANs", above.

### Q: I ran "ovs-ofctl add-flow br0 nw_dst=192.168.0.1,actions=drop"
   but I got a funny message like this:

       ofp_util|INFO|normalization changed ofp_match, details:
       ofp_util|INFO| pre: nw_dst=192.168.0.1
       ofp_util|INFO|post:

   and when I ran "ovs-ofctl dump-flows br0" I saw that my nw_dst
   match had disappeared, so that the flow ends up matching every
   packet.

A: The term "normalization" in the log message means that a flow
   cannot match on an L3 field without saying what L3 protocol is in
   use.  The "ovs-ofctl" command above didn't specify an L3 protocol,
   so the L3 field match was dropped.

   In this case, the L3 protocol could be IP or ARP.  A correct
   command for each possibility is, respectively:

       ovs-ofctl add-flow br0 ip,nw_dst=192.168.0.1,actions=drop

   and 

       ovs-ofctl add-flow br0 arp,nw_dst=192.168.0.1,actions=drop

   Similarly, a flow cannot match on an L4 field without saying what
   L4 protocol is in use.  For example, the flow match "tp_src=1234"
   is, by itself, meaningless and will be ignored.  Instead, to match
   TCP source port 1234, write "tcp,tp_src=1234", or to match UDP
   source port 1234, write "udp,tp_src=1234".

### Q: How can I figure out the OpenFlow port number for a given port?

A: The OFPT_FEATURES_REQUEST message requests an OpenFlow switch to
   respond with an OFPT_FEATURES_REPLY that, among other information,
   includes a mapping between OpenFlow port names and numbers.  From a
   command prompt, "ovs-ofctl show br0" makes such a request and
   prints the response for switch br0.

   The Interface table in the Open vSwitch database also maps OpenFlow
   port names to numbers.  To print the OpenFlow port number
   associated with interface eth0, run:

       ovs-vsctl get Interface eth0 ofport

   You can print the entire mapping with:

       ovs-vsctl -- --columns=name,ofport list Interface

   but the output mixes together interfaces from all bridges in the
   database, so it may be confusing if more than one bridge exists.

   In the Open vSwitch database, ofport value -1 means that the
   interface could not be created due to an error.  (The Open vSwitch
   log should indicate the reason.)  ofport value [] (the empty set)
   means that the interface hasn't been created yet.  The latter is
   normally an intermittent condition (unless ovs-vswitchd is not
   running).

### Q: I added some flows with my controller or with ovs-ofctl, but when I
   run "ovs-dpctl dump-flows" I don't see them.

A: ovs-dpctl queries a kernel datapath, not an OpenFlow switch.  It
   won't display the information that you want.  You want to use
   "ovs-ofctl dump-flows" instead.

### Q: It looks like each of the interfaces in my bonded port shows up
   as an individual OpenFlow port.  Is that right?

A: Yes, Open vSwitch makes individual bond interfaces visible as
   OpenFlow ports, rather than the bond as a whole.  The interfaces
   are treated together as a bond for only a few purposes:

   - Sending a packet to the OFPP_NORMAL port.  (When an OpenFlow
     controller is not configured, this happens implicitly to
     every packet.)

   - Mirrors configured for output to a bonded port.

   It would make a lot of sense for Open vSwitch to present a bond as
   a single OpenFlow port.  If you want to contribute an
   implementation of such a feature, please bring it up on the Open
   vSwitch development mailing list at dev@openvswitch.org.

### Q: I have a sophisticated network setup involving Open vSwitch, VMs or
   multiple hosts, and other components.  The behavior isn't what I
   expect.  Help!

A: To debug network behavior problems, trace the path of a packet,
   hop-by-hop, from its origin in one host to a remote host.  If
   that's correct, then trace the path of the response packet back to
   the origin.

   The open source tool called "plotnetcfg" can help to understand the
   relationship between the networking devices on a single host.

   Usually a simple ICMP echo request and reply ("ping") packet is
   good enough.  Start by initiating an ongoing "ping" from the origin
   host to a remote host.  If you are tracking down a connectivity
   problem, the "ping" will not display any successful output, but
   packets are still being sent.  (In this case the packets being sent
   are likely ARP rather than ICMP.)

   Tools available for tracing include the following:

   - "tcpdump" and "wireshark" for observing hops across network
     devices, such as Open vSwitch internal devices and physical
     wires.

   - "ovs-appctl dpif/dump-flows <br>" in Open vSwitch 1.10 and
     later or "ovs-dpctl dump-flows <br>" in earlier versions.
     These tools allow one to observe the actions being taken on
     packets in ongoing flows.

     See ovs-vswitchd(8) for "ovs-appctl dpif/dump-flows"
     documentation, ovs-dpctl(8) for "ovs-dpctl dump-flows"
     documentation, and "Why are there so many different ways to
     dump flows?" above for some background.

   - "ovs-appctl ofproto/trace" to observe the logic behind how
     ovs-vswitchd treats packets.  See ovs-vswitchd(8) for
     documentation.  You can out more details about a given flow
     that "ovs-dpctl dump-flows" displays, by cutting and pasting
     a flow from the output into an "ovs-appctl ofproto/trace"
     command.

   - SPAN, RSPAN, and ERSPAN features of physical switches, to
     observe what goes on at these physical hops.

   Starting at the origin of a given packet, observe the packet at
   each hop in turn.  For example, in one plausible scenario, you
   might:

   1. "tcpdump" the "eth" interface through which an ARP egresses
      a VM, from inside the VM.

   2. "tcpdump" the "vif" or "tap" interface through which the ARP
      ingresses the host machine.

   3. Use "ovs-dpctl dump-flows" to spot the ARP flow and observe
      the host interface through which the ARP egresses the
      physical machine.  You may need to use "ovs-dpctl show" to
      interpret the port numbers.  If the output seems surprising,
      you can use "ovs-appctl ofproto/trace" to observe details of
      how ovs-vswitchd determined the actions in the "ovs-dpctl
      dump-flows" output.

   4. "tcpdump" the "eth" interface through which the ARP egresses
      the physical machine.

   5. "tcpdump" the "eth" interface through which the ARP
      ingresses the physical machine, at the remote host that
      receives the ARP.

   6. Use "ovs-dpctl dump-flows" to spot the ARP flow on the
      remote host that receives the ARP and observe the VM "vif"
      or "tap" interface to which the flow is directed.  Again,
      "ovs-dpctl show" and "ovs-appctl ofproto/trace" might help.

   7. "tcpdump" the "vif" or "tap" interface to which the ARP is
      directed.

   8. "tcpdump" the "eth" interface through which the ARP
      ingresses a VM, from inside the VM.

   It is likely that during one of these steps you will figure out the
   problem.  If not, then follow the ARP reply back to the origin, in
   reverse.

### Q: How do I make a flow drop packets?

A: To drop a packet is to receive it without forwarding it.  OpenFlow
   explicitly specifies forwarding actions.  Thus, a flow with an
   empty set of actions does not forward packets anywhere, causing
   them to be dropped.  You can specify an empty set of actions with
   "actions=" on the ovs-ofctl command line.  For example:

       ovs-ofctl add-flow br0 priority=65535,actions=

   would cause every packet entering switch br0 to be dropped.

   You can write "drop" explicitly if you like.  The effect is the
   same.  Thus, the following command also causes every packet
   entering switch br0 to be dropped:

       ovs-ofctl add-flow br0 priority=65535,actions=drop

   "drop" is not an action, either in OpenFlow or Open vSwitch.
   Rather, it is only a way to say that there are no actions.

### Q: I added a flow to send packets out the ingress port, like this:

       ovs-ofctl add-flow br0 in_port=2,actions=2

   but OVS drops the packets instead.

A: Yes, OpenFlow requires a switch to ignore attempts to send a packet
   out its ingress port.  The rationale is that dropping these packets
   makes it harder to loop the network.  Sometimes this behavior can
   even be convenient, e.g. it is often the desired behavior in a flow
   that forwards a packet to several ports ("floods" the packet).

   Sometimes one really needs to send a packet out its ingress port
   ("hairpin"). In this case, output to OFPP_IN_PORT, which in
   ovs-ofctl syntax is expressed as just "in_port", e.g.:

       ovs-ofctl add-flow br0 in_port=2,actions=in_port

   This also works in some circumstances where the flow doesn't match
   on the input port.  For example, if you know that your switch has
   five ports numbered 2 through 6, then the following will send every
   received packet out every port, even its ingress port:

       ovs-ofctl add-flow br0 actions=2,3,4,5,6,in_port

   or, equivalently:

       ovs-ofctl add-flow br0 actions=all,in_port

   Sometimes, in complicated flow tables with multiple levels of
   "resubmit" actions, a flow needs to output to a particular port
   that may or may not be the ingress port.  It's difficult to take
   advantage of OFPP_IN_PORT in this situation.  To help, Open vSwitch
   provides, as an OpenFlow extension, the ability to modify the
   in_port field.  Whatever value is currently in the in_port field is
   the port to which outputs will be dropped, as well as the
   destination for OFPP_IN_PORT.  This means that the following will
   reliably output to port 2 or to ports 2 through 6, respectively:

       ovs-ofctl add-flow br0 in_port=2,actions=load:0->NXM_OF_IN_PORT[],2
       ovs-ofctl add-flow br0 actions=load:0->NXM_OF_IN_PORT[],2,3,4,5,6

   If the input port is important, then one may save and restore it on
   the stack:

        ovs-ofctl add-flow br0 actions=push:NXM_OF_IN_PORT[],\
                                       load:0->NXM_OF_IN_PORT[],\
                                       2,3,4,5,6,\
                                       pop:NXM_OF_IN_PORT[]

### Q: My bridge br0 has host 192.168.0.1 on port 1 and host 192.168.0.2
   on port 2.  I set up flows to forward only traffic destined to the
   other host and drop other traffic, like this:

      priority=5,in_port=1,ip,nw_dst=192.168.0.2,actions=2
      priority=5,in_port=2,ip,nw_dst=192.168.0.1,actions=1
      priority=0,actions=drop

   But it doesn't work--I don't get any connectivity when I do this.
   Why?

A: These flows drop the ARP packets that IP hosts use to establish IP
   connectivity over Ethernet.  To solve the problem, add flows to
   allow ARP to pass between the hosts:

      priority=5,in_port=1,arp,actions=2
      priority=5,in_port=2,arp,actions=1

   This issue can manifest other ways, too.  The following flows that
   match on Ethernet addresses instead of IP addresses will also drop
   ARP packets, because ARP requests are broadcast instead of being
   directed to a specific host:

      priority=5,in_port=1,dl_dst=54:00:00:00:00:02,actions=2
      priority=5,in_port=2,dl_dst=54:00:00:00:00:01,actions=1
      priority=0,actions=drop

   The solution already described above will also work in this case.
   It may be better to add flows to allow all multicast and broadcast
   traffic:

      priority=5,in_port=1,dl_dst=01:00:00:00:00:00/01:00:00:00:00:00,actions=2
      priority=5,in_port=2,dl_dst=01:00:00:00:00:00/01:00:00:00:00:00,actions=1

### Q: My bridge disconnects from my controller on add-port/del-port.

A: Reconfiguring your bridge can change your bridge's datapath-id because
   Open vSwitch generates datapath-id from the MAC address of one of its ports.
   In that case, Open vSwitch disconnects from controllers because there's
   no graceful way to notify controllers about the change of datapath-id.

   To avoid the behaviour, you can configure datapath-id manually.

      ovs-vsctl set bridge br0 other-config:datapath-id=0123456789abcdef

### Q: My controller is getting errors about "buffers".  What's going on?

A: When a switch sends a packet to an OpenFlow controller using a
   "packet-in" message, it can also keep a copy of that packet in a
   "buffer", identified by a 32-bit integer "buffer_id".  There are
   two advantages to buffering.  First, when the controller wants to
   tell the switch to do something with the buffered packet (with a
   "packet-out" OpenFlow request), it does not need to send another
   copy of the packet back across the OpenFlow connection, which
   reduces the bandwidth cost of the connection and improves latency.
   This enables the second advantage: the switch can optionally send
   only the first part of the packet to the controller (assuming that
   the switch only needs to look at the first few bytes of the
   packet), further reducing bandwidth and improving latency.

   However, buffering introduces some issues of its own.  First, any
   switch has limited resources, so if the controller does not use a
   buffered packet, the switch has to decide how long to keep it
   buffered.  When many packets are sent to a controller and buffered,
   Open vSwitch can discard buffered packets that the controller has
   not used after as little as 5 seconds.  This means that
   controllers, if they make use of packet buffering, should use the
   buffered packets promptly.  (This includes sending a "packet-out"
   with no actions if the controller does not want to do anything with
   a buffered packet, to clear the packet buffer and effectively
   "drop" its packet.)

   Second, packet buffers are one-time-use, meaning that a controller
   cannot use a single packet buffer in two or more "packet-out"
   commands.  Open vSwitch will respond with an error to the second
   and subsequent "packet-out"s in such a case.

   Finally, a common error early in controller development is to try
   to use buffer_id 0 in a "packet-out" message as if 0 represented
   "no buffered packet".  This is incorrect usage: the buffer_id with
   this meaning is actually 0xffffffff.

   ovs-vswitchd(8) describes some details of Open vSwitch packet
   buffering that the OpenFlow specification requires implementations
   to document.


Development
-----------

### Q: How do I implement a new OpenFlow message?

A: Add your new message to "enum ofpraw" and "enum ofptype" in
   lib/ofp-msgs.h, following the existing pattern.  Then recompile and
   fix all of the new warnings, implementing new functionality for the
   new message as needed.  (If you configure with --enable-Werror, as
   described in [INSTALL.md], then it is impossible to miss any warnings.)

   If you need to add an OpenFlow vendor extension message for a
   vendor that doesn't yet have any extension messages, then you will
   also need to edit build-aux/extract-ofp-msgs.

### Q: How do I add support for a new field or header?

A: Add new members for your field to "struct flow" in lib/flow.h, and
   add new enumerations for your new field to "enum mf_field_id" in
   lib/meta-flow.h, following the existing pattern.  Also, add support
   to miniflow_extract() in lib/flow.c for extracting your new field
   from a packet into struct miniflow.  Then recompile and fix all of
   the new warnings, implementing new functionality for the new field
   or header as needed.  (If you configure with --enable-Werror, as
   described in [INSTALL.md], then it is impossible to miss any
   warnings.)

   If you want kernel datapath support for your new field, you also
   need to modify the kernel module for the operating systems you are
   interested in.  This isn't mandatory, since fields understood only
   by userspace work too (with a performance penalty), so it's
   reasonable to start development without it.  If you implement
   kernel module support for Linux, then the Linux kernel "netdev"
   mailing list is the place to submit that support first; please read
   up on the Linux kernel development process separately.  The Windows
   datapath kernel module support, on the other hand, is maintained
   within the OVS tree, so patches for that can go directly to
   ovs-dev.

### Q: How do I add support for a new OpenFlow action?

A: Add your new action to "enum ofp_raw_action_type" in
   lib/ofp-actions.c, following the existing pattern.  Then recompile
   and fix all of the new warnings, implementing new functionality for
   the new action as needed.  (If you configure with --enable-Werror,
   as described in [INSTALL.md], then it is impossible to miss any
   warnings.)

   If you need to add an OpenFlow vendor extension action for a vendor
   that doesn't yet have any extension actions, then you will also
   need to edit build-aux/extract-ofp-actions.


Contact 
-------

bugs@openvswitch.org
http://openvswitch.org/

[PORTING.md]:PORTING.md
[WHY-OVS.md]:WHY-OVS.md
[INSTALL.md]:INSTALL.md
[OPENFLOW-1.1+.md]:OPENFLOW-1.1+.md
[INSTALL.DPDK.md]:INSTALL.DPDK.md