Comment 7 for bug 1290486

I've been able to track down what I believe is the root problem.

If ovsdb-server (run by the openvswitch-switch service) restarts, the neutron-openvswitch-agent loses its connection and needs to be manually restarted in order to reconnect.

Causes of this bug I've seen have included ovsdb-server segfaulting, being kill -9ed, and being gracefully restarted with "service openvswitch-switch restart".

The errors recorded in /var/log/upstart/neutron-openvswitch-agent.log vary depending on why ovsdb-server went away:

2014-03-23 20:10:01.883 20375 ERROR neutron.agent.linux.ovsdb_monitor [req-a776b981-b86b-4437-ab65-0c6be6070094 None] Error received from ovsdb monitor: ovsdb-client: unix:/var/run/openvswitch/db.sock: receive failed (End of file)
2014-03-24 01:40:17.617 20375 ERROR neutron.agent.linux.ovsdb_monitor [req-a776b981-b86b-4437-ab65-0c6be6070094 None] Error received from ovsdb monitor: 2014-03-24T01:40:17Z|00001|fatal_signal|WARN|terminating with signal 15 (Terminated)
2014-03-24 04:08:59.718 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: ovsdb-client: unix:/var/run/openvswitch/db.sock: receive failed (End of file)
2014-03-24 22:44:22.174 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: ovsdb-client: unix:/var/run/openvswitch/db.sock: receive failed (End of file)
2014-03-24 22:44:52.220 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: ovsdb-client: failed to connect to "unix:/var/run/openvswitch/db.sock" (Connection refused)
2014-03-24 22:45:22.266 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: ovsdb-client: failed to connect to "unix:/var/run/openvswitch/db.sock" (Connection refused)
2014-03-24 22:45:52.310 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: ovsdb-client: failed to connect to "unix:/var/run/openvswitch/db.sock" (Connection refused)
2014-03-24 22:46:22.355 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: ovsdb-client: failed to connect to "unix:/var/run/openvswitch/db.sock" (Connection refused)
2014-03-24 22:49:27.179 8455 ERROR neutron.agent.linux.ovsdb_monitor [req-d2c2cbd5-a77a-4455-84ac-0a8ec69b41e8 None] Error received from ovsdb monitor: 2014-03-24T22:49:27Z|00001|fatal_signal|WARN|terminating with signal 15 (Terminated)
2014-03-24 22:55:45.441 16033 ERROR neutron.agent.linux.ovsdb_monitor [req-5fe682ce-138e-46d6-aa7e-f0d43ab576ee None] Error received from ovsdb monitor: ovsdb-client: unix:/var/run/openvswitch/db.sock: receive failed (End of file)

In all cases, the result is the same: until neutron-openvswitch-agent is restarted, no traffic is passed onto the tapXXXXX interface inside the dhcp-XXXXX netns