In the context of OpenStack networking, I was trying to explore Open vSwitch. I felt it’s better to go one step back, and try with a pure libvirt guest before I try it with OpenStack networking.
On why Open vSwitch compared to regular Linux bridge?
- In short (as Thomas Graf, Kernel networking subsystem developer, put it) — Software Defined Networking(SDN)
- Open vSwitch’s upstream documentation provides a more detailed explanation.
Here’s a simple scenario, where the machine in test has a single physical NIC, obtaining its IP address from DHCP. And, running KVM guests managed via libvirt.
Install Open vSwitch
Install the Open vSwitch package (this is on Fedora 19):
$ yum install openvswitch -y
Enable the openvswitch systemd unit file, and start the daemon:
$ systemctl enable openvswitch.service $ systemctl start openvswitch.service
Check the status Open vSwitch service, to ensure it’s ‘Active’:
$ systemctl status openvswitch.service
Configure Open vSwitch (OVS) bridge
Before you proceed, ensure to have physical access or access via serial console to the machine, because, associating a physical interface with an Open vSwitch bridge will result in lost connectivity.
Reasoning is here under the ‘Configuration problems’ section.
Add an OVS bridge device:
$ ovs-vsctl add-br ovsbr0
Associate the OVS bridge device to eth0 (or em1). (At this point, network connectivity will be lost.)
$ ovs-vsctl add-port ovsbr0 eth0
I was obtaining IP address to my host from DHCP, so I first cleared it from the physical interface, and associated it with the Open vSwitch bridge device (ovsbr0).
$ ifconfig eth0 0.0.0.0 $ ifconfig ovsbr0 10.xx.yyy.zzz
I killed the existing
dhclient instance on ‘eth0’, and initiated it on ovsbr0:
$ dhclient ovsbr0 &
List the OVS database contents
$ ovs-vsctl show 3dc7f3e3-5872-47c0-ba6f-1cb12065f4d0 Bridge "ovsbr0" Port "eth0" Interface "eth0" Port "ovsbr0" Interface "ovsbr0" type: internal ovs_version: "1.10.0"
Update libvirt guest’s bridge source
I have an existing KVM guest, managed by libvirt, with its default network source associated with libvirt’s ‘virbr0). Let’s modify its network to Open vSwitch bridge.
Edit the libvirt’s guest XML
$ virsh edit f18vm
The attribute should look as below (take note of the highlighted attributes):
[...] <interface type='bridge'> <mac address='52:54:00:fb:00:01'/> <source bridge='ovsbr0'/> <virtualport type='openvswitch'/> <model type='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> </interface> [...]
Once the guest XML is edited and saved, dump its contents to stdout, you’ll notice an additional attribute
interfaceid added automatically:
$ virsh dumpxml f18vm | grep bridge -A8 <interface type='bridge'> <mac address='52:54:00:fb:00:01'/> <source bridge='ovsbr0'/> <virtualport type='openvswitch'> <parameters interfaceid='74b6858e-8012-4caa-85c7-b64902a19605'/> </virtualport> <model type='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> </interface> <serial type='pty'> <target port='0'/>
Start the guest, and check if it’s IP address matches the host subnet:
$ virsh start fed18vm --console $ ifconfig eth0