We had to investigate the operation of one of our Openstack compute nodes as it was exhibiting some unusual behaviour. We quickly determined that there was some unexpected packet loss and we had reason to believe that this could have been due to the packet processing in the node. Investigating this problem necessitated some deeper exploration of how packets are processed in the node, particularly relating to the mix of ovs bridges, linux bridges and iptables. It turns out that this is rather complex and clear information describing how all this fits together in detail is not readily available. Here, we note what we learnt from this exploration.