In a default configuration of Xen there will be a virtual Ethernet device created for each interface which will be associated with a bridge. A previous post documented how to configure a bridge named xenbr0.
The basic configuration of Xen that most people use is to have a single virtual Ethernet port for each Xen instance and have them all connected to the one bridge, and then the Dom0 will have an IP address on the bridge interface that is used for routing packets to the outside world. This works really well if you have a subnet that you are using for all Xen DomU IP addresses, if you are using NAT for communication, or if the DomU needs no communication outside the Dom0 and other DomU’s on the same machine (a common case for testing).
But if you have a collection of servers that you want to consolidate on a single piece of hardware then you end up using a single sub-net that spans some physical machines, some Xen Dom0’s, and some DomU’s. The solution to this is to use bridged networking.
Unfortunately most documentation of bridged networking is really confusing, and non of my google searches turned up the most relevant fact:
When setting up a bridge on the local Ethernet you must make your physical ethernet device (eth0 or whatever) be strictly a slave to the bridge and then assign the IP address used for the physical network to the bridge.
ifconfig eth0 up brctl addif xenbr0 eth0
For example if you have 10.0.0.42 being the IP address used by the Dom0 on the local Ethernet via device eth0 and you want to use bridging for DomU’s then you simply make eth0 owned by xenbr0 (the typical name for the Xen bridge) with the above commands in your script to configure the xenbr0 device. Then treat xenbr0 in the same way that you treated eth0 before enabling bridging.
Also there’s nothing stopping you from having one bridge for DomU’s that can talk directly to the physical Ethernet and another for DomU’s that are only to use routed networking, see my previous post about using multiple ethernet devices in Xen for more background information.