this is obsolete doc -- see http://doc.nethence.com/ instead

XEN -- Bridge network configuration 

Dom0 

To use different bridges on each network interface, create a custom wrapper, 

cd /etc/xen/scripts/

vi network-custombr

like, 

#!/bin/ksh

set -e

[[ $1 != start && $1 != stop ]] && print "start|stop" && exit 1

/etc/xen/scripts/network-bridge $1 bridge=bridge0 netdev=eth0 vifnum=0

fix perms, 

chmod +x network-custombr

Note. this might also help if you enabled Netfilter, 

#iptables -A FORWARD -m physdev --physdev-in eth0 ! --physdev-out eth0 -j ACCEPT

#iptables -A FORWARD -m physdev --physdev-out eth0 ! --physdev-in eth0 -j ACCEPT

And point the xend config to it, 

cd /etc/xen/

vi xend-config.sxp

change, 

(network-script custombr)

(vif-script vif-bridge)

Note. disable ip forwarding, 

cd /etc/

mv sysctl.conf sysctl.conf.dist

sed '/^$/d; /^#/d' sysctl.conf.dist > sysctl.conf

vi /etc/sysctl.conf

change, 

net.ipv4.ip_forward=0

net.ipv4.conf.all.proxy_arp=0

net.ipv4.conf.default.proxy_arp=0

and apply, 

sysctl -p

DomU 

Either use the first available bridge, 

vif = [ 'vifname=vifname' ]

or choose a specific bridge, 

vif = [ 'vifname=vifname,bridge=bridgename' ]

References 

http://docwhat.org/2008/08/xen-networking/