Using Open vSwitch with libvirt and QEMU/KVM

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本文介绍如何配置OpenvSwitch (OVS) 与 libvirt 和 QEMU/KVM 的结合使用，实现虚拟机网络接口自动加入指定VLAN的功能。首先需要安装最新版OVS以避免已知问题，并配置桥接兼容模式。接下来创建对应每个所需VLAN的虚拟桥接器，然后通过编辑libvirt中的虚拟机设置将虚拟机连接到相应的VLAN。

Using Open vSwitch with libvirt and QEMU/KVM

This is a quick and dirty write-up of using Open vSwitch(OVS) with libvirt and QEMU/KVM. I assume you already have libvirt( http://libvirt.org) up and running with QEMU/KVM. Other hypervisors may work but I haven't tried them at this time.

Grab the master of Open vSwitch( http://openvswitch.org/) with git because in previous versions there is an issue with removing ports from a fake bridge when the TAP is just deleted without first removing it from the bridge. This is what happens with libvirt and KVM when a VM is shutdown. This is fixed in the OVS master. The fix will also be in versions 1.2.2, 1.3.0, and later when released.

1) Compile and install OVS by following INSTALL.Linux

2) Compile and install OVS bridge compatibility mode by following INSTALL.bridge

Now you should have a working OVS install with bridge compatibility mode.

3) Create a fake bridge for each VLAN you want available. So say you have br0 as your real bridge and you want a VLAN 101.

ovs-vsctl add-br br101 br0 101

This will create a fake bridge tagged with VLAN 101. Do this for every VLAN you want.

4) In libvirt use a bridge interface setting the source to the appropriate fake bridge.

For example if you want a virtual machine on VLAN 101 assign it to br101

You can add the network interface by editing domain's XML directly with virsh.

<interface type='bridge'>
<source bridge='br101'/>
</interface>

Add this in the <devices> section. Libvirt will fill in the <mac/> and <address/>.

You can also add the network interface with virt-manager.

Now when you boot the VM libvirt will create the TAP interface and add it to br101. Since bridge compatibility mode is running OVS will add the port tagged with the appropriate VLAN when libvirt adds the interface to the bridge.

After starting the VM you should see the associated TAP interface when you list the ports in OVS.

ovs-vsctl list port

It should have the appropriate tag. In this case the tag should be 101.

Notice the port will not show up under br0 if you list-ports. You will have to list-ports on the fake bridge

ovs-vsctl list-ports br101

When you shutdown the VM the TAP interface will be destroyed and the port will be removed from OVS. If the TAP interface is destroyed, but the port still exists in OVS then you are not using a new enough version of OVS.

This will not help with mirrors or other advanced features but will give you the basic ability to automatically add and remove ports on the virtual switch with the appropriate VLAN tags for your virtual machines' interfaces.

I am working on a helper service that will enable the ability to automatically handle mirrors and other advanced functionality. It will also allow you to avoid using bridge compatibility mode if you desire. Eventually I hope to see OVS support built into libvirt but this will help until then.

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