Open vSwitch 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, 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 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 1.4.x. 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: - Bridge compatibility, that is, support for the ovs-brcompatd daemon that (if you enable it) lets "brctl" and other Linux bridge tools transparently work with Open vSwitch instead. We do not expect bridge compatibility to ever be available in upstream Linux. If you need bridge compatibility, use the kernel module from the Open vSwitch distribution instead of the upstream Linux kernel module. - 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 module. - 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. 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: Is there any documentation on the database tables and fields? A: Yes. ovs-vswitchd.conf.db(5) is a comprehensive reference. VLANs ----- 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: 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. This will only be suitable for some situations, though. Controllers ----------- Q: What versions of OpenFlow does Open vSwitch support? A: Open vSwitch supports OpenFlow 1.0. It also includes a number of extensions that bring many of the features from later versions of OpenFlow. Work is underway to provide support for later versions and can be tracked here: http://openvswitch.org/development/openflow-1-x-plan/ Q: I'm getting "error type 45250 code 0". What's that? A: This is a Open vSwitch extension to OpenFlow error codes. Open vSwitch uses this extension when it must report an error to an OpenFlow controller but no standard OpenFlow error code is suitable. Open vSwitch logs the errors that it sends to controllers, so the easiest thing to do is probably to look at the ovs-vswitchd log to find out what the error was. If you want to dissect the extended error message yourself, the format is documented in include/openflow/nicira-ext.h in the Open vSwitch source distribution. The extended error codes are documented in lib/ofp-errors.h. Q1: Some of the traffic that I'd expect my OpenFlow controller to see doesn't actually appear through the OpenFlow connection, even though I know that it's going through. Q2: Some of the OpenFlow flows that my controller sets up don't seem to apply to certain traffic, especially traffic between OVS and the controller itself. A: By default, Open vSwitch assumes that OpenFlow controllers are connected "in-band", that is, that the controllers are actually part of the network that is being controlled. In in-band mode, Open vSwitch sets up special "hidden" flows to make sure that traffic can make it back and forth between OVS and the controllers. These hidden flows are higher priority than any flows that can be set up through OpenFlow, and they are not visible through normal OpenFlow flow table dumps. Usually, the hidden flows are desirable and helpful, but occasionally they can cause unexpected behavior. You can view the full OpenFlow flow table, including hidden flows, on bridge br0 with the command: ovs-appctl bridge/dump-flows br0 to help you debug. The hidden flows are those with priorities greater than 65535 (the maximum priority that can be set with OpenFlow). The DESIGN file at the top level of the Open vSwitch source distribution describes the in-band model in detail. If your controllers are not actually in-band (e.g. they are on localhost via 127.0.0.1, or on a separate network), then you should configure your controllers in "out-of-band" mode. If you have one controller on bridge br0, then you can configure out-of-band mode on it with: ovs-vsctl set controller br0 connection-mode=out-of-band Q: I configured all my controllers for out-of-band control mode but "ovs-appctl bridge/dump-flows" still shows some hidden flows. A: You probably have a remote manager configured (e.g. with "ovs-vsctl set-manager"). By default, Open vSwitch assumes that managers need in-band rules set up on every bridge. You can disable these rules on bridge br0 with: ovs-vsctl set bridge br0 other-config:disable-in-band=true This actually disables in-band control entirely for the bridge, as if all the bridge's controllers were configured for out-of-band control. Q: My OpenFlow controller doesn't see the VLANs that I expect. A: See answer under "VLANs", above. Q: I ran "ovs-ofctl add-flow br0 nw_dst=192.168.0.1,actions=drop" but I got a funny message like this: ofp_util|INFO|normalization changed ofp_match, details: ofp_util|INFO| pre: nw_dst=192.168.0.1 ofp_util|INFO|post: and when I ran "ovs-ofctl dump-flows br0" I saw that my nw_dst match had disappeared, so that the flow ends up matching every packet. A: The term "normalization" in the log message means that a flow cannot match on an L3 field without saying what L3 protocol is in use. The "ovs-ofctl" command above didn't specify an L3 protocol, so the L3 field match was dropped. In this case, the L3 protocol could be IP or ARP. A correct command for each possibility is, respectively: ovs-ofctl add-flow br0 ip,nw_dst=192.168.0.1,actions=drop and ovs-ofctl add-flow br0 arp,nw_dst=192.168.0.1,actions=drop Similarly, a flow cannot match on an L4 field without saying what L4 protocol is in use. For example, the flow match "tp_src=1234" is, by itself, meaningless and will be ignored. Instead, to match TCP source port 1234, write "tcp,tp_src=1234", or to match UDP source port 1234, write "udp,tp_src=1234". Q: How can I figure out the OpenFlow port number for a given port? A: The OFPT_FEATURES_REQUEST message requests an OpenFlow switch to respond with an OFPT_FEATURES_REPLY that, among other information, includes a mapping between OpenFlow port names and numbers. From a command prompt, "ovs-ofctl show br0" makes such a request and prints the response for switch br0. The Interface table in the Open vSwitch database also maps OpenFlow port names to numbers. To print the OpenFlow port number associated with interface eth0, run: ovs-vsctl get Interface eth0 ofport You can print the entire mapping with: ovs-vsctl -- --columns=name,ofport list Interface but the output mixes together interfaces from all bridges in the database, so it may be confusing if more than one bridge exists. In the Open vSwitch database, ofport value -1 means that the interface could not be created due to an error. (The Open vSwitch log should indicate the reason.) ofport value [] (the empty set) means that the interface hasn't been created yet. The latter is normally an intermittent condition (unless ovs-vswitchd is not running). Contact ------- bugs@openvswitch.org http://openvswitch.org/