I think I've found it. My HP Microserver running Ubuntu 12.04 has two network cards in: eth0 and eth2. I run the bridge with a configuration targetting eth0, and my router has the IP address for eth0 mapped via port 4711 to the outside world, but when I checked the NAT sessions table on the router I could see sessions for both eth0 and eth2. As soon as I ran $ ifconfig eth2 down I could see on SLAVE adjacency up messages. So there is something fishy going on with multiple network cards, might be worth trying to get to the bottom of this and then documenting what the issue is. Mark.