1 Open vSwitch <http://openvswitch.org>
3 Frequently Asked Questions
4 ==========================
9 Q: What is Open vSwitch?
11 A: Open vSwitch is a production quality open source software switch
12 designed to be used as a vswitch in virtualized server environments. A
13 vswitch forwards traffic between different VMs on the same physical host
14 and also forwards traffic between VMs and the physical network. Open
15 vSwitch supports standard management interfaces (e.g. sFlow, NetFlow,
16 RSPAN, CLI), and is open to programmatic extension and control using
17 OpenFlow and the OVSDB management protocol.
19 Open vSwitch as designed to be compatible with modern switching
20 chipsets. This means that it can be ported to existing high-fanout
21 switches allowing the same flexible control of the physical
22 infrastructure as the virtual infrastructure. It also means that
23 Open vSwitch will be able to take advantage of on-NIC switching
24 chipsets as their functionality matures.
26 Q: What virtualization platforms can use Open vSwitch?
28 A: Open vSwitch can currently run on any Linux-based virtualization
29 platform (kernel 2.6.18 and newer), including: KVM, VirtualBox, Xen,
30 Xen Cloud Platform, XenServer. As of Linux 3.3 it is part of the
31 mainline kernel. The bulk of the code is written in platform-
32 independent C and is easily ported to other environments. We welcome
33 inquires about integrating Open vSwitch with other virtualization
36 Q: How can I try Open vSwitch?
38 A: The Open vSwitch source code can be built on a Linux system. You can
39 build and experiment with Open vSwitch on any Linux machine.
40 Packages for various Linux distributions are available on many
41 platforms, including: Debian, Ubuntu, Fedora.
43 You may also download and run a virtualization platform that already
44 has Open vSwitch integrated. For example, download a recent ISO for
45 XenServer or Xen Cloud Platform. Be aware that the version
46 integrated with a particular platform may not be the most recent Open
49 Q: Does Open vSwitch only work on Linux?
51 A: No, Open vSwitch has been ported to a number of different operating
52 systems and hardware platforms. Most of the development work occurs
53 on Linux, but the code should be portable to any POSIX system. We've
54 seen Open vSwitch ported to a number of different platforms,
55 including FreeBSD, Windows, and even non-POSIX embedded systems.
57 By definition, the Open vSwitch Linux kernel module only works on
58 Linux and will provide the highest performance. However, a userspace
59 datapath is available that should be very portable.
61 Q: What's involved with porting Open vSwitch to a new platform or
64 A: The PORTING document describes how one would go about porting Open
65 vSwitch to a new operating system or hardware platform.
67 Q: Why would I use Open vSwitch instead of the Linux bridge?
69 A: Open vSwitch is specially designed to make it easier to manage VM
70 network configuration and monitor state spread across many physical
71 hosts in dynamic virtualized environments. Please see WHY-OVS for a
72 more detailed description of how Open vSwitch relates to the Linux
75 Q: How is Open vSwitch related to distributed virtual switches like the
76 VMware vNetwork distributed switch or the Cisco Nexus 1000V?
78 A: Distributed vswitch applications (e.g., VMware vNetwork distributed
79 switch, Cisco Nexus 1000V) provide a centralized way to configure and
80 monitor the network state of VMs that are spread across many physical
81 hosts. Open vSwitch is not a distributed vswitch itself, rather it
82 runs on each physical host and supports remote management in a way
83 that makes it easier for developers of virtualization/cloud
84 management platforms to offer distributed vswitch capabilities.
86 To aid in distribution, Open vSwitch provides two open protocols that
87 are specially designed for remote management in virtualized network
88 environments: OpenFlow, which exposes flow-based forwarding state,
89 and the OVSDB management protocol, which exposes switch port state.
90 In addition to the switch implementation itself, Open vSwitch
91 includes tools (ovs-controller, ovs-ofctl, ovs-vsctl) that developers
92 can script and extend to provide distributed vswitch capabilities
93 that are closely integrated with their virtualization management
96 Q: Why doesn't Open vSwitch support distribution?
98 A: Open vSwitch is intended to be a useful component for building
99 flexible network infrastructure. There are many different approaches
100 to distribution which balance trade-offs between simplicity,
101 scalability, hardware compatibility, convergence times, logical
102 forwarding model, etc. The goal of Open vSwitch is to be able to
103 support all as a primitive building block rather than choose a
104 particular point in the distributed design space.
106 Q: How can I contribute to the Open vSwitch Community?
108 A: You can start by joining the mailing lists and helping to answer
109 questions. You can also suggest improvements to documentation. If
110 you have a feature or bug you would like to work on, send a mail to
111 one of the mailing lists:
113 http://openvswitch.org/mlists/
120 Q: What does it mean for an Open vSwitch release to be LTS (long-term
123 A: All official releases have been through a comprehensive testing
124 process and are suitable for production use. Planned releases will
125 occur several times a year. If a significant bug is identified in an
126 LTS release, we will provide an updated release that includes the
127 fix. Releases that are not LTS may not be fixed and may just be
128 supplanted by the next major release. The current LTS release is
131 Q: What features are not available in the Open vSwitch kernel datapath
132 that ships as part of the upstream Linux kernel?
134 A: The kernel module in upstream Linux 3.3 and later does not include
135 the following features:
137 - Bridge compatibility, that is, support for the ovs-brcompatd
138 daemon that (if you enable it) lets "brctl" and other Linux
139 bridge tools transparently work with Open vSwitch instead.
141 We do not expect bridge compatibility to ever be available in
142 upstream Linux. If you need bridge compatibility, use the
143 kernel module from the Open vSwitch distribution instead of the
144 upstream Linux kernel module.
146 - Tunnel virtual ports, that is, interfaces with type "gre",
147 "ipsec_gre", "capwap". It is possible to create tunnels in
148 Linux and attach them to Open vSwitch as system devices.
149 However, they cannot be dynamically created through the OVSDB
150 protocol or set the tunnel ids as a flow action.
152 Work is in progress in adding these features to the upstream
153 Linux version of the Open vSwitch kernel module. For now, if
154 you need these features, use the kernel module from the Open
155 vSwitch distribution instead of the upstream Linux kernel
158 - Patch virtual ports, that is, interfaces with type "patch".
159 You can use Linux "veth" devices as a substitute.
161 We don't have any plans to add patch ports upstream.
163 Q: What features are not available when using the userspace datapath?
165 A: Tunnel and patch virtual ports are not supported, as described in the
166 previous answer. It is also not possible to use queue-related
167 actions. On Linux kernels before 2.6.39, maximum-sized VLAN packets
168 may not be transmitted.
171 Configuration Problems
172 ----------------------
174 Q: I created a bridge and added my Ethernet port to it, using commands
178 ovs-vsctl add-port br0 eth0
180 and as soon as I ran the "add-port" command I lost all connectivity
183 A: A physical Ethernet device that is part of an Open vSwitch bridge
184 should not have an IP address. If one does, then that IP address
185 will not be fully functional.
187 You can restore functionality by moving the IP address to an Open
188 vSwitch "internal" device, such as the network device named after
189 the bridge itself. For example, assuming that eth0's IP address is
190 192.168.128.5, you could run the commands below to fix up the
193 ifconfig eth0 0.0.0.0
194 ifconfig br0 192.168.128.5
196 (If your only connection to the machine running OVS is through the
197 IP address in question, then you would want to run all of these
198 commands on a single command line, or put them into a script.) If
199 there were any additional routes assigned to eth0, then you would
200 also want to use commands to adjust these routes to go through br0.
202 If you use DHCP to obtain an IP address, then you should kill the
203 DHCP client that was listening on the physical Ethernet interface
204 (e.g. eth0) and start one listening on the internal interface
205 (e.g. br0). You might still need to manually clear the IP address
206 from the physical interface (e.g. with "ifconfig eth0 0.0.0.0").
208 There is no compelling reason why Open vSwitch must work this way.
209 However, this is the way that the Linux kernel bridge module has
210 always worked, so it's a model that those accustomed to Linux
211 bridging are already used to. Also, the model that most people
212 expect is not implementable without kernel changes on all the
213 versions of Linux that Open vSwitch supports.
215 By the way, this issue is not specific to physical Ethernet
216 devices. It applies to all network devices except Open vswitch
219 Q: I created a bridge and added a couple of Ethernet ports to it,
220 using commands like these:
223 ovs-vsctl add-port br0 eth0
224 ovs-vsctl add-port br0 eth1
226 and now my network seems to have melted: connectivity is unreliable
227 (even connectivity that doesn't go through Open vSwitch), all the
228 LEDs on my physical switches are blinking, wireshark shows
229 duplicated packets, and CPU usage is very high.
231 A: More than likely, you've looped your network. Probably, eth0 and
232 eth1 are connected to the same physical Ethernet switch. This
233 yields a scenario where OVS receives a broadcast packet on eth0 and
234 sends it out on eth1, then the physical switch connected to eth1
235 sends the packet back on eth0, and so on forever. More complicated
236 scenarios, involving a loop through multiple switches, are possible
239 The solution depends on what you are trying to do:
241 - If you added eth0 and eth1 to get higher bandwidth or higher
242 reliability between OVS and your physical Ethernet switch,
243 use a bond. The following commands create br0 and then add
244 eth0 and eth1 as a bond:
247 ovs-vsctl add-bond br0 bond0 eth0 eth1
249 Bonds have tons of configuration options. Please read the
250 documentation on the Port table in ovs-vswitchd.conf.db(5)
253 - Perhaps you don't actually need eth0 and eth1 to be on the
254 same bridge. For example, if you simply want to be able to
255 connect each of them to virtual machines, then you can put
256 each of them on a bridge of its own:
259 ovs-vsctl add-port br0 eth0
262 ovs-vsctl add-port br1 eth1
264 and then connect VMs to br0 and br1. (A potential
265 disadvantage is that traffic cannot directly pass between br0
266 and br1. Instead, it will go out eth0 and come back in eth1,
269 - If you have a redundant or complex network topology and you
270 want to prevent loops, turn on spanning tree protocol (STP).
271 The following commands create br0, enable STP, and add eth0
272 and eth1 to the bridge. The order is important because you
273 don't want have to have a loop in your network even
277 ovs-vsctl set bridge br0 stp_enable=true
278 ovs-vsctl add-port br0 eth0
279 ovs-vsctl add-port br0 eth1
281 The Open vSwitch implementation of STP is not well tested.
282 Please report any bugs you observe, but if you'd rather avoid
283 acting as a beta tester then another option might be your
286 Q: I can't seem to use Open vSwitch in a wireless network.
288 A: Wireless base stations generally only allow packets with the source
289 MAC address of NIC that completed the initial handshake.
290 Therefore, without MAC rewriting, only a single device can
291 communicate over a single wireless link.
293 This isn't specific to Open vSwitch, it's enforced by the access
294 point, so the same problems will show up with the Linux bridge or
295 any other way to do bridging.
297 Q: Is there any documentation on the database tables and fields?
299 A: Yes. ovs-vswitchd.conf.db(5) is a comprehensive reference.
307 A: Many drivers in Linux kernels before version 3.3 had VLAN-related
308 bugs. If you are having problems with VLANs that you suspect to be
309 driver related, then you have several options:
311 - Upgrade to Linux 3.3 or later.
313 - Build and install a fixed version of the particular driver
314 that is causing trouble, if one is available.
316 - Use a NIC whose driver does not have VLAN problems.
318 - Use "VLAN splinters", a feature in Open vSwitch 1.4 and later
319 that works around bugs in kernel drivers. To enable VLAN
320 splinters on interface eth0, use the command:
322 ovs-vsctl set interface eth0 other-config:enable-vlan-splinters=true
324 For VLAN splinters to be effective, Open vSwitch must know
325 which VLANs are in use. See the "VLAN splinters" section in
326 the Interface table in ovs-vswitchd.conf.db(5) for details on
327 how Open vSwitch infers in-use VLANs.
329 VLAN splinters increase memory use and reduce performance, so
330 use them only if needed.
332 - Apply the "vlan workaround" patch from the XenServer kernel
333 patch queue, build Open vSwitch against this patched kernel,
334 and then use ovs-vlan-bug-workaround(8) to enable the VLAN
335 workaround for each interface whose driver is buggy.
337 (This is a nontrivial exercise, so this option is included
338 only for completeness.)
340 It is not always easy to tell whether a Linux kernel driver has
341 buggy VLAN support. The ovs-vlan-test(8) and ovs-test(8) utilities
342 can help you test. See their manpages for details. Of the two
343 utilities, ovs-test(8) is newer and more thorough, but
344 ovs-vlan-test(8) may be easier to use.
346 Q: VLANs still don't work. I've tested the driver so I know that it's OK.
348 A: Do you have VLANs enabled on the physical switch that OVS is
349 attached to? Make sure that the port is configured to trunk the
350 VLAN or VLANs that you are using with OVS.
352 Q: Outgoing VLAN-tagged traffic goes through OVS to my physical switch
353 and to its destination host, but OVS seems to drop incoming return
356 A: It's possible that you have the VLAN configured on your physical
357 switch as the "native" VLAN. In this mode, the switch treats
358 incoming packets either tagged with the native VLAN or untagged as
359 part of the native VLAN. It may also send outgoing packets in the
360 native VLAN without a VLAN tag.
362 If this is the case, you have two choices:
364 - Change the physical switch port configuration to tag packets
365 it forwards to OVS with the native VLAN instead of forwarding
368 - Change the OVS configuration for the physical port to a
369 native VLAN mode. For example, the following sets up a
370 bridge with port eth0 in "native-tagged" mode in VLAN 9:
373 ovs-vsctl add-port br0 eth0 tag=9 vlan_mode=native-tagged
375 In this situation, "native-untagged" mode will probably work
376 equally well. Refer to the documentation for the Port table
377 in ovs-vswitchd.conf.db(5) for more information.
379 Q: Can I configure an IP address on a VLAN?
381 A: Yes. Use an "internal port" configured as an access port. For
382 example, the following configures IP address 192.168.0.7 on VLAN 9.
383 That is, OVS will forward packets from eth0 to 192.168.0.7 only if
384 they have an 802.1Q header with VLAN 9. Conversely, traffic
385 forwarded from 192.168.0.7 to eth0 will be tagged with an 802.1Q
389 ovs-vsctl add-port br0 eth0
390 ovs-vsctl add-port br0 vlan9 tag=9 -- set interface vlan9 type=internal
391 ifconfig vlan9 192.168.0.7
393 Q: My OpenFlow controller doesn't see the VLANs that I expect.
395 A: The configuration for VLANs in the Open vSwitch database (e.g. via
396 ovs-vsctl) only affects traffic that goes through Open vSwitch's
397 implementation of the OpenFlow "normal switching" action. By
398 default, when Open vSwitch isn't connected to a controller and
399 nothing has been manually configured in the flow table, all traffic
400 goes through the "normal switching" action. But, if you set up
401 OpenFlow flows on your own, through a controller or using ovs-ofctl
402 or through other means, then you have to implement VLAN handling
405 You can use "normal switching" as a component of your OpenFlow
406 actions, e.g. by putting "normal" into the lists of actions on
407 ovs-ofctl or by outputting to OFPP_NORMAL from an OpenFlow
408 controller. This will only be suitable for some situations,
411 Q: I configured ports on a bridge as access ports with different VLAN
415 ovs-vsctl set-controller br0 tcp:192.168.0.10:6633
416 ovs-vsctl add-port br0 eth0
417 ovs-vsctl add-port br0 tap0 tag=9
418 ovs-vsctl add-port br0 tap1 tag=10
420 but the VMs running behind tap0 and tap1 can still communicate,
421 that is, they are not isolated from each other even though they are
424 A: Do you have a controller configured on br0 (as the commands above
425 do)? If so, then this is a variant on the previous question, "My
426 OpenFlow controller doesn't see the VLANs that I expect," and you
427 can refer to the answer there for more information.
433 Q: What versions of OpenFlow does Open vSwitch support?
435 A: Open vSwitch supports OpenFlow 1.0. It also includes a number of
436 extensions that bring many of the features from later versions of
437 OpenFlow. Work is underway to provide support for later versions and
440 http://openvswitch.org/development/openflow-1-x-plan/
442 Q: I'm getting "error type 45250 code 0". What's that?
444 A: This is a Open vSwitch extension to OpenFlow error codes. Open
445 vSwitch uses this extension when it must report an error to an
446 OpenFlow controller but no standard OpenFlow error code is
449 Open vSwitch logs the errors that it sends to controllers, so the
450 easiest thing to do is probably to look at the ovs-vswitchd log to
451 find out what the error was.
453 If you want to dissect the extended error message yourself, the
454 format is documented in include/openflow/nicira-ext.h in the Open
455 vSwitch source distribution. The extended error codes are
456 documented in lib/ofp-errors.h.
458 Q1: Some of the traffic that I'd expect my OpenFlow controller to see
459 doesn't actually appear through the OpenFlow connection, even
460 though I know that it's going through.
461 Q2: Some of the OpenFlow flows that my controller sets up don't seem
462 to apply to certain traffic, especially traffic between OVS and
463 the controller itself.
465 A: By default, Open vSwitch assumes that OpenFlow controllers are
466 connected "in-band", that is, that the controllers are actually
467 part of the network that is being controlled. In in-band mode,
468 Open vSwitch sets up special "hidden" flows to make sure that
469 traffic can make it back and forth between OVS and the controllers.
470 These hidden flows are higher priority than any flows that can be
471 set up through OpenFlow, and they are not visible through normal
472 OpenFlow flow table dumps.
474 Usually, the hidden flows are desirable and helpful, but
475 occasionally they can cause unexpected behavior. You can view the
476 full OpenFlow flow table, including hidden flows, on bridge br0
479 ovs-appctl bridge/dump-flows br0
481 to help you debug. The hidden flows are those with priorities
482 greater than 65535 (the maximum priority that can be set with
485 The DESIGN file at the top level of the Open vSwitch source
486 distribution describes the in-band model in detail.
488 If your controllers are not actually in-band (e.g. they are on
489 localhost via 127.0.0.1, or on a separate network), then you should
490 configure your controllers in "out-of-band" mode. If you have one
491 controller on bridge br0, then you can configure out-of-band mode
494 ovs-vsctl set controller br0 connection-mode=out-of-band
496 Q: I configured all my controllers for out-of-band control mode but
497 "ovs-appctl bridge/dump-flows" still shows some hidden flows.
499 A: You probably have a remote manager configured (e.g. with "ovs-vsctl
500 set-manager"). By default, Open vSwitch assumes that managers need
501 in-band rules set up on every bridge. You can disable these rules
504 ovs-vsctl set bridge br0 other-config:disable-in-band=true
506 This actually disables in-band control entirely for the bridge, as
507 if all the bridge's controllers were configured for out-of-band
510 Q: My OpenFlow controller doesn't see the VLANs that I expect.
512 A: See answer under "VLANs", above.
514 Q: I ran "ovs-ofctl add-flow br0 nw_dst=192.168.0.1,actions=drop"
515 but I got a funny message like this:
517 ofp_util|INFO|normalization changed ofp_match, details:
518 ofp_util|INFO| pre: nw_dst=192.168.0.1
521 and when I ran "ovs-ofctl dump-flows br0" I saw that my nw_dst
522 match had disappeared, so that the flow ends up matching every
525 A: The term "normalization" in the log message means that a flow
526 cannot match on an L3 field without saying what L3 protocol is in
527 use. The "ovs-ofctl" command above didn't specify an L3 protocol,
528 so the L3 field match was dropped.
530 In this case, the L3 protocol could be IP or ARP. A correct
531 command for each possibility is, respectively:
533 ovs-ofctl add-flow br0 ip,nw_dst=192.168.0.1,actions=drop
537 ovs-ofctl add-flow br0 arp,nw_dst=192.168.0.1,actions=drop
539 Similarly, a flow cannot match on an L4 field without saying what
540 L4 protocol is in use. For example, the flow match "tp_src=1234"
541 is, by itself, meaningless and will be ignored. Instead, to match
542 TCP source port 1234, write "tcp,tp_src=1234", or to match UDP
543 source port 1234, write "udp,tp_src=1234".
545 Q: How can I figure out the OpenFlow port number for a given port?
547 A: The OFPT_FEATURES_REQUEST message requests an OpenFlow switch to
548 respond with an OFPT_FEATURES_REPLY that, among other information,
549 includes a mapping between OpenFlow port names and numbers. From a
550 command prompt, "ovs-ofctl show br0" makes such a request and
551 prints the response for switch br0.
553 The Interface table in the Open vSwitch database also maps OpenFlow
554 port names to numbers. To print the OpenFlow port number
555 associated with interface eth0, run:
557 ovs-vsctl get Interface eth0 ofport
559 You can print the entire mapping with:
561 ovs-vsctl -- --columns=name,ofport list Interface
563 but the output mixes together interfaces from all bridges in the
564 database, so it may be confusing if more than one bridge exists.
566 In the Open vSwitch database, ofport value -1 means that the
567 interface could not be created due to an error. (The Open vSwitch
568 log should indicate the reason.) ofport value [] (the empty set)
569 means that the interface hasn't been created yet. The latter is
570 normally an intermittent condition (unless ovs-vswitchd is not
577 http://openvswitch.org/
projects / openvswitch / blob