While debugging an issue with OpenStack and floating ips, I ran into a strange issue where running tcpdump on the bridge interface on the network controller would cause packets to be forwarded successfully to a compute node, but if I stopped running tcpdump than the packets wouldn’t get forwarded.
Somebody on serverfault provided the solution: tcpdump puts the interface into promiscuous mode. And, indeed, if I set the interface into promiscuous mode, the packets got forwarded. This is a classic Heisenbug.
Lorin's Tech Blog 