How to connect different virtual machines to different networks in a Xen virtualization environment with bridged networking setup using 802.1q VLAN support provided by Linux.

Intro

Using a single network interface and a network appliance that support 801.1q VLAN trunking to separate
different virtual machines running on a single phisical host to different network segments is possibile and quiet with VLAN support.

Xen networking relies on standard networking tools used by common GNU/Linux distributions: ip, ifconfig, brctl (bridge-utils)

These are the steps needed to implement the solution:

1. use the 8021q module to handle vlan traffic in dom0
2. create a bridge for each VLAN the dom0 see
3. attach the network interface of the virtual machines to the correct bridge

1) 8021q module

It's important to load the 8021q module at boot time adding the next line to the /etc/modules file:

8021q

2) Networking configuration in dom0

Firstly make sure that Xen doesn't modify your network configuration specifing "network-dummy" per "network-script" in the /etc/xen/xend-config.sxp configuration file.

For each VLAN, bring up a logical network interface for the transport in /etc/network/interfaces:

auto vlan183
iface eth0.vlan180 inet manual
        vlan-raw-device eth0

For each VLAN, bring up a bridge adding the following lines to the /etc/network/interfaces configuration file:

auto xenbr183
iface xenbr183 inet manual
        bridge_ports    eth0.183
        bridge_stp off

3) Networking configuration in domUs

Specify the bridge you want to use in the configuration file of the vm for each vif you've assigned:

...
vif         = [ 'ip=10.20.198.8,mac=00:16:3E:12:FF:69,bridge=xenbr183' ]
...