[libvirt] existing bridge and no iptable rules

hi, i try to setup libvirt with an existing network setup (bridge with NO physical device, routes and iptables are already configured, dhcp named ..) . this bridge should be available for the libvirt domains. now the problem: every configuration i tried ends in iptable rules beeing created which mess with my network setup (e.g. i try to route vpn traffic from a guest to another network outside) is there a way of defining a virtual network / network source / bridge for domains that takes an existing bridge and does no setup ( ip, iptables, .. )? thanks, markus