ovs-ofctl: Document masked versions of arp_sha and arp_tha matches

[ovs.git] / FAQ

                 Open vSwitch <http://openvswitch.org>

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

Q: How can I try Open vSwitch?

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

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

Q: Does Open vSwitch only work on Linux?

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

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

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

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

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

A: Open vSwitch is specially designed to make it easier to manage VM
   network configuration and monitor state spread across many physical
   hosts in dynamic virtualized environments.  Please see WHY-OVS 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/


Releases
--------

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

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

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

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

   Open vSwitch   Linux kernel
   ------------   -------------
       1.4.x      2.6.18 to 3.2
       1.5.x      2.6.18 to 3.2
       1.6.x      2.6.18 to 3.2
       1.7.x      2.6.18 to 3.3
       1.8.x      2.6.18 to 3.4
       1.9.x      2.6.18 to 3.8
       1.10.x     2.6.18 to 3.8
       1.11.x     2.6.18 to 3.8
       2.0.x      2.6.32 to 3.10
       2.1.x      2.6.32 to 3.11

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

A: IPFIX flow monitoring requires the Linux kernel module from Open
   vSwitch version 1.10.90 or later.

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

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

Q: What 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 3.3 and later does not include
   tunnel virtual ports, that is, interfaces with type "gre",
   "ipsec_gre", "gre64", "ipsec_gre64", "vxlan", or "lisp".  It is
   possible to create tunnels in Linux and attach them to Open vSwitch
   as system devices.  However, they cannot be dynamically created
   through the OVSDB protocol or set the tunnel ids as a flow action.

   Work is in progress in adding tunnel virtual ports to the upstream
   Linux version of the Open vSwitch kernel module.  For now, if you
   need these features, use the kernel module from the Open vSwitch
   distribution instead of the upstream Linux kernel module.

   The upstream kernel module does not include patch ports, but this
   only matters for Open vSwitch 1.9 and earlier, because Open vSwitch
   1.10 and later implement patch ports without using this kernel
   feature.

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

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

Q: What 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 VLAN as an RSPAN VLAN, that is, enable
   mirroring of all traffic to that VLAN?

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

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

   To later disable mirroring, run:

       ovs-vsctl clear bridge br0 mirrors

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

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

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

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

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

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

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

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

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

   To later disable mirroring and destroy the GRE tunnel:

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

Q: Does Open vSwitch support ERSPAN?

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

Q: How do I connect two bridges?

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

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

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

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

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

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

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 "exact-match"
        or "datapath" or "kernel" flows, do not support wildcards or
        priorities and comprise only a single table, which makes them
        suitable for caching.   OpenFlow flows and exact-match flows
        also support different actions and number ports differently.

        Exact-match 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 exact-match flows.

  Each of the commands for dumping flows has a different purpose:

      - "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 exact-match 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 exact-match flows for only the specified bridge,
        regardless of the type.


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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

Q: How do I configure Quality of Service (QoS)?

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: 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 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).


VLANs
-----

Q: What's a VLAN?

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

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

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

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

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

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

Q: VLANs don't work.

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

       - Upgrade to Linux 3.3 or later.

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

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

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

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

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

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

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

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

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

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

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

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

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

   If this is the case, you have two choices:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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:192.168.0.10:6633
       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.


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
   the IETF draft available here:

   http://tools.ietf.org/html/draft-mahalingam-dutt-dcops-vxlan-03

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

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

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

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

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


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

Q: What versions of OpenFlow does Open vSwitch support?

A: Open vSwitch 1.9 and earlier support only OpenFlow 1.0 (plus
   extensions that bring in many of the features from later versions
   of OpenFlow).

   Open vSwitch 1.10 and later have experimental support for OpenFlow
   1.2 and 1.3.  On these versions of Open vSwitch, the following
   command enables OpenFlow 1.0, 1.2, and 1.3 on bridge br0:

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

   Open vSwitch version 1.12 and later will have experimental support
   for OpenFlow 1.1, 1.2, and 1.3.  On these versions of Open vSwitch,
   the following command enables OpenFlow 1.0, 1.1, 1.2, and 1.3 on
   bridge br0:

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

   Use the -O option to enable support for later versions of OpenFlow
   in ovs-ofctl.  For example:

       ovs-ofctl -O OpenFlow13 dump-flows br0

   Support for OpenFlow 1.1, 1.2, and 1.3 is still incomplete.  Work
   to be done is tracked in OPENFLOW-1.1+ in the Open vSwitch sources
   (also via http://openvswitch.org/development/openflow-1-x-plan/).
   When support for a given OpenFlow version is solidly implemented,
   Open vSwitch will enable that version by default.

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

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

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

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

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

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

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

       ovs-appctl bridge/dump-flows br0

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

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

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

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

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

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

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

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

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

A: See answer under "VLANs", above.

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

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

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

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

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

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

   and 

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

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

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

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

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

       ovs-vsctl get Interface eth0 ofport

   You can print the entire mapping with:

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

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

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

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

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

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

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

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

       - Mirrors configured for output to a bonded port.

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

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

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

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

   Tools available for tracing include the following:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Q: How do I make a flow drop packets?

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

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

   would cause every packet entering switch br0 to be dropped.

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

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

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

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

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

   but OVS drops the packets instead.

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

   Sometimes one really needs to send a packet out its ingress port.
   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

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