Neutron ovs エージェントで Agent out of sync with plugin! メッセージが発生する

Issue

• 以下のようなメッセージが表示されることがありますが、何が原因となっているのかが分かりません。

2016-02-07 06:04:03.095 224435 ERROR neutron.plugins.openvswitch.agent.ovs_neutron_agent [-] Error while processing VIF ports
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent Traceback (most recent call last):
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent   File "/usr/lib/python2.7/site-packages/neutron/plugins/openvswitch/agent/ovs_neutron_agent.py", line 1342, in rpc_loop
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent     ovs_restarted)
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent   File "/usr/lib/python2.7/site-packages/neutron/plugins/openvswitch/agent/ovs_neutron_agent.py", line 1146, in process_network_ports
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent     port_info.get('updated', set()))
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent   File "/usr/lib/python2.7/site-packages/neutron/agent/securitygroups_rpc.py", line 259, in setup_port_filters
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent     self.refresh_firewall()
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent   File "/usr/lib/python2.7/site-packages/neutron/agent/securitygroups_rpc.py", line 226, in refresh_firewall
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent     for device in devices.values():
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent AttributeError:'NoneType' object has no attribute 'values'
2016-02-07 06:04:03.095 224435 TRACE neutron.plugins.openvswitch.agent.ovs_neutron_agent
2016-02-07 06:04:03.096 224435 INFO neutron.plugins.openvswitch.agent.ovs_neutron_agent [-] Agent out of sync with plugin!