Comment 3 for bug 1293540

I'm pretty sure that this is a race condition between neutron linuxbridge agent, that will periodically look for empty bridges and remove them, and nova, which will remove and reattach the interface, but doesn't expect the bridge being deleted in between. The race condition involved is that the neutron cleanup job must run exactly in the interval between removal and reattach.

This also happens when rebooting an instance, see https://bugs.launchpad.net/neutron/+bug/1328546 which seems to be the same underlying issue.