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Open vSwitch <>
Frequently Asked Questions
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,
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 2.6.18 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
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
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-controller, ovs-ofctl, ovs-vsctl) that developers
can script and extend to provide distributed vswitch capabilities
that are closely integrated with their virtualization management
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:
Q: What does it mean for an Open vSwitch release to be LTS (long-term
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
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 3.7
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 2.6.18
and later.
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
the following features:
- Tunnel virtual ports, that is, interfaces with type "gre",
"ipsec_gre", "capwap". 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 these features 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
- Patch virtual ports, that is, interfaces with type "patch".
You can use Linux "veth" devices as a substitute.
We don't have any plans to add patch ports upstream.
Q: What features are not available when using the userspace datapath?
A: Tunnel and patch 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: 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: 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
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; 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= \
-- --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
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, you could run the commands below to fix up the
ifconfig eth0
ifconfig br0
(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").
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
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
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: 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: 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
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
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
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: 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 on VLAN 9.
That is, OVS will forward packets from eth0 to only if
they have an 802.1Q header with VLAN 9. Conversely, traffic
forwarded from 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
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
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. This will only be suitable for some situations,
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:
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: 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 versions 1.10 and later will 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
Support for OpenFlow 1.1 is incomplete enough that it cannot yet be
enabled, even experimentally.
Support for OpenFlow 1.2 and 1.3 is still incomplete. Work to be
done is tracked in OPENFLOW-1.1+ in the Open vSwitch source tree
(also via
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
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
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, 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
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=,actions=drop"
but I got a funny message like this:
ofp_util|INFO|normalization changed ofp_match, details:
ofp_util|INFO| pre: nw_dst=
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
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=,actions=drop
ovs-ofctl add-flow br0 arp,nw_dst=,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
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.)
- The "autopath" Nicira extension action. However, "autopath"
is deprecated and scheduled for removal in February 2013.
- 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
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