Comment 4 for bug 1623664

The recommended solution is to disable or delete Ubuntu cron job of neutron-netns-cleanup.
neutron-netns-cleanup command searches network namespaces only with a loopback interface and deletes such namespaces. This potentially creates a situation where undesired netns deletion occurs.

I think the cron job was added when the neutron default behavior was not to delete namespaces. it was long ago.

In addition, Ubuntu package registers three neutron-netns-cleanup cron jobs (for dhcp-agent, l3-agent, lbaas-agent), but all of three do the same job. This also looks tricky.