Mine

auto eth0 xen-br0

iface eth0 inet manual
        up ifconfig eth0 up
        up ethtool -K eth0 tx off || true
        down ifconfig eth0 down

iface xen-br0 inet manual
        up brctl addbr xen-br0
        up brctl addif xen-br0 eth0
        up ifconfig xen-br0 up
        up ip addr add 192.168.1.100/24 broadcast 192.168.1.255 dev xen-br0
        up ip route add default via 192.168.1.254 dev xen-br0
        down ip route flush dev xen-br0
        down ip addr flush dev xen-br0
        down ifconfig xen-br0 down
        down brctl delbr xen-br0