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FAQ
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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-controller, 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
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 possbility 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.
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: 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 [email protected].
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: