take a look at this two points: 1.- the vSwitch has to be configured in promiscuos 2.- only assign one external NIC to the vSwitch

also explore this two commands in order to follow the packets as it goes in openstack 1.- tcpdump of each involved interfaces: eth, bridge, tap 2.- brctl to show how interfaces are beeing bridged, and brctl showmacs to see how the mac are associated to portIDs (maybe you can see there when MACs are flapping betweens portIDs, and that would be a loop if you have 2 external NICs in a vSwitch)