path: root/FAQ.rst

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

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 3.10 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 document <PORTING.rst>`__ 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.  Refer to `WHY-OVS <WHY-OVS.rst>`__
    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 occur twice 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.

    For more information on the Open vSwitch release process, refer to `release
    process <Documentation/release-process.md>`__.

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 4.3
    2.6.x        3.10 to 4.7
    ============ ==============

    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: Are all features available with all datapaths?

    A: Open vSwitch supports different datapaths on different platforms.  Each
    datapath has a different feature set: the following tables try to summarize
    the status.

    Supported datapaths:

    Linux upstream
      The datapath implemented by the kernel module shipped with Linux
      upstream.  Since features have been gradually introduced into the kernel,
      the table mentions the first Linux release whose OVS module supports the
      feature.

    Linux OVS tree
      The datapath implemented by the Linux kernel module distributed with the
      OVS source tree.

    Userspace
      Also known as DPDK, dpif-netdev or dummy datapath. It is the only
      datapath that works on NetBSD, FreeBSD and Mac OSX.

    Hyper-V
      Also known as the Windows datapath.

    The following table lists the datapath supported features from an Open
    vSwitch user's perspective.

    ===================== ============== ============== ========= =======
    Feature               Linux upstream Linux OVS tree Userspace Hyper-V
    ===================== ============== ============== ========= =======
    NAT                   4.6            YES            NO        NO
    Connection tracking   4.3            YES            PARTIAL   PARTIAL
    Tunnel - LISP         NO             YES            NO        NO
    Tunnel - STT          NO             YES            NO        YES
    Tunnel - GRE          3.11           YES            YES       YES
    Tunnel - VXLAN        3.12           YES            YES       YES
    Tunnel - Geneve       3.18           YES            YES       YES
    Tunnel - GRE-IPv6     NO             NO             YES       NO
    Tunnel - VXLAN-IPv6   4.3            YES            YES       NO
    Tunnel - Geneve-IPv6  4.4            YES            YES       NO
    QoS - Policing        YES            YES            YES       NO
    QoS - Shaping         YES            YES            NO        NO
    sFlow                 YES            YES            YES       NO
    IPFIX                 3.10           YES            YES       NO
    Set action            YES            YES            YES       PARTIAL
    NIC Bonding           YES            YES            YES       NO
    Multiple VTEPs        YES            YES            YES       NO
    ===================== ============== ============== ========= =======

    Do note, however:

    * Only a limited set of flow fields is modifiable via the set action by the
      Hyper-V datapath.
    * The Hyper-V datapath only supports one physical NIC per datapath. This is
      why bonding is not supported.
    * The Hyper-V datapath can have at most one IP address configured as a
      tunnel endpoint.

    The following table lists features that do not *directly* impact an Open
    vSwitch user, e.g. because their absence can be hidden by the ofproto layer
    (usually this comes with a performance penalty).

    ===================== ============== ============== ========= =======
    Feature               Linux upstream Linux OVS tree Userspace Hyper-V
    ===================== ============== ============== ========= =======
    SCTP flows            3.12           YES            YES       YES
    MPLS                  3.19           YES            YES       YES
    UFID                  4.0            YES            YES       NO
    Megaflows             3.12           YES            YES       NO
    Masked set action     4.0            YES            YES       NO
    Recirculation         3.19           YES            YES       YES
    TCP flags matching    3.13           YES            YES       NO
    Validate flow actions YES            YES            N/A       NO
    Multiple datapaths    YES            YES            YES       NO
    Tunnel TSO - STT      N/A            YES            NO        YES
    ===================== ============== ============== ========= =======

Q: What DPDK version does each Open vSwitch release work with?

    A: The following table lists the DPDK version against which the given
    versions of Open vSwitch will successfully build.

    ============ =====
    Open vSwitch DPDK
    ============ =====
    2.2.x        1.6
    2.3.x        1.6
    2.4.x        2.0
    2.5.x        2.2
    2.6.x        16.07
    ============ =====

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

    A. 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 other-config:dpdk-init
    configuration in the database and will display lines with "EAL:..." during
    startup when other_config:dpdk-init is set to 'true'.

    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 <INSTALL.DPDK.rst>`__ 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-vsctl del-port br0 gre0

Q: Does Open vSwitch support ERSPAN?

    A: No.  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 <INSTALL.DPDK.rst>`__.

    - 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.  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 the `installation guide
    <INSTALL.rst>`__, 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.  The OVS conntrack
    feature (see the "ct" action in ovs-ofctl(8)) can implement a stateful
    firewall.

    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).

Q: I observe packet loss at the beginning of RFC2544 tests on a server running
few hundred container apps bridged to OVS with traffic generated by HW traffic
generator.  How can I fix this?

    A: This is expected behavior on virtual switches.  RFC2544 tests were
    designed for hardware switches, which don't have caches on the fastpath
    that need to be heated.  Traffic generators in order to prime the switch
    use learning phase to heat the caches before sending the actual traffic in
    test phase.  In case of OVS the cache is flushed quickly and to accommodate
    the traffic generator's delay between learning and test phase, the max-idle
    timeout settings should be changed to 50000 ms.::

        $ ovs-vsctl --no-wait set Open_vSwitch . other_config:max-idle=50000

Q: How can I configure the bridge internal interface MTU? Why does Open vSwitch
keep changing internal ports MTU?

    A: By default Open vSwitch overrides the internal interfaces (e.g. br0)
    MTU.  If you have just an internal interface (e.g. br0) and a physical
    interface (e.g. eth0), then every change in MTU to eth0 will be reflected
    to br0.  Any manual MTU configuration using `ip` or `ifconfig` on internal
    interfaces is going to be overridden by Open vSwitch to match the current
    bridge minimum.

    Sometimes this behavior is not desirable, for example with tunnels.  The
    MTU of an internal interface can be explicitly set using the following
    command:::

        $ ovs-vsctl set int br0 mtu_request=1450

    After this, Open vSwitch will configure br0 MTU to 1450.  Since this
    setting is in the database it will be persistent (compared to what happens
    with `ip` or `ifconfig`).

    The MTU configuration can be removed to restore the default behavior
    with:::

        $ ovs-vsctl set int br0 mtu_request=[]

    The mtu_request column can be used to configure MTU even for physical
    interfaces (e.g. eth0).

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 not 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=8000

    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 upto 2.5 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"
    <https://tools.ietf.org/html/rfc1122#section-3.3.4.2>`__ 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, e.g. 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
--------------

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 OF1.6
    =============== ===== ===== ===== ===== ===== ===== =====
    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   (*)   (*)   ---
    2.4              yes   yes   yes   yes   (*)   (*)   ---
    2.5              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.

    Some versions of OpenFlow are supported with missing features and therefore
    not enabled by default: OpenFlow 1.4 and 1.5, in Open vSwitch 2.3 and
    later, as well as OpenFlow 1.6 in Open vSwitch 2.5 and later.  Also, the
    OpenFlow 1.6 specification is still under development and thus subject to
    change.

    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.)

    The `OpenFlow guide <OPENFLOW.rst>`__ 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
    include/openvswitch/ofp-errors.h.

Q: 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.

    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: 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: See above.

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
       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 complains that OVS is not buffering packets.
What's going on?

    A: "Packet buffering" is an optional OpenFlow feature, and controllers
    should detect how many "buffers" an OpenFlow switch implements.  It was
    recently noticed that OVS implementation of the buffering feature was not
    compliant to OpenFlow specifications.  Rather than fix it and risk
    controller incompatibility, the buffering feature is removed as of OVS 2.7.
    Controllers are already expected to work properly in cases where the switch
    can not buffer packets, but sends full packets in "packet-in" messages
    instead, so this change should not affect existing users.  After the change
    OVS always sends the ``buffer_id`` as ``0xffffffff`` in "packet-in"
    messages and will send an error response if any other value of this field
    is included in a "packet-out" or a "flow mod" sent by a controller.

Q: How does OVS divide flows among buckets in an OpenFlow "select" group?

    A: In Open vSwitch 2.3 and earlier, Open vSwitch used the destination
    Ethernet address to choose a bucket in a select group.

    Open vSwitch 2.4 and later by default hashes the source and destination
    Ethernet address, VLAN ID, Ethernet type, IPv4/v6 source and destination
    address and protocol, and for TCP and SCTP only, the source and destination
    ports.  The hash is "symmetric", meaning that exchanging source and
    destination addresses does not change the bucket selection.

    Select groups in Open vSwitch 2.4 and later can be configured to use a
    different hash function, using a Netronome extension to the OpenFlow 1.5+
    group_mod message.  For more information, see
    Documentation/group-selection-method-property.txt in the Open vSwitch
    source tree.  (OpenFlow 1.5 support in Open vSwitch is still experimental.)

Q: I added a flow to accept packets on VLAN 123 and output them on VLAN 456,
like so:::

    $ ovs-ofctl add-flow br0 dl_vlan=123,actions=output:1,mod_vlan_vid:456

but the packets are actually being output in VLAN 123.  Why?

    A: OpenFlow actions are executed in the order specified.  Thus, the actions
    above first output the packet, then change its VLAN.  Since the output
    occurs before changing the VLAN, the change in VLAN will have no visible
    effect.

    To solve this and similar problems, order actions so that changes to
    headers happen before output, e.g.:::

        $ ovs-ofctl add-flow br0 dl_vlan=123,actions=mod_vlan_vid:456,output:1

Q: The "learn" action can't learn the action I want, can you improve it?

    A: By itself, the "learn" action can only put two kinds of actions into the
    flows that it creates: "load" and "output" actions.  If "learn" is used in
    isolation, these are severe limits.

    However, "learn" is not meant to be used in isolation.  It is a primitive
    meant to be used together with other Open vSwitch features to accomplish a
    task.  Its existing features are enough to accomplish most tasks.

    Here is an outline of a typical pipeline structure that allows for
    versatile behavior using "learn":

    - Flows in table A contain a "learn" action, that populates flows in table
      L, that use a "load" action to populate register R with information about
      what was learned.

    - Flows in table B contain two sequential resubmit actions: one to table L
      and another one to table B+1.

    - Flows in table B+1 match on register R and act differently depending on
      what the flows in table L loaded into it.

    This approach can be used to implement many "learn"-based features.  For
    example:

    - Resubmit to a table selected based on learned information, e.g. see:
      http://openvswitch.org/pipermail/discuss/2016-June/021694.html

    - MAC learning in the middle of a pipeline, as described in `the tutorial
      <tutorial/Tutorial.md>`__.

    - TCP state based firewalling, by learning outgoing connections based on
      SYN packets and matching them up with incoming packets.

    - At least some of the features described in T. A. Hoff, "Extending Open
      vSwitch to Facilitate Creation of Stateful SDN Applications".

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 the
    `installation guide <INSTALL.rst>`__, 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, and to ``nx_put_raw()`` in
    ``lib/nx-match.c`` to output your new field in OXM matches.  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 the `installation guide <INSTALL.rst>`__, 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 the `installation guide <INSTALL.rst>`__, 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``.