Comment 7 for bug 1953165

I had the same symptoms as this issue, with slightly different steps to reproduce.

On an existing wallaby deployment using neutron ML2/OVS, create a VLAN network & IPv4 subnet. The DHCP agent logs show the same backtrace as in the original description. Restarting the DHCP agent shows the same backtrace each time.

While DHCP seems to work, metadata does not.

There are two other networks which do not exhibit this issue.

Worked around the issue as suggested:

ip netns # find ID of network
ip netns exec qdhcp-dee9459f-7ed8-4627-9c42-4006ec09d5fd bash
ip a | grep dadfailed
ip a del fe80::a9fe:a9fe/64 dev tapf0cd099d-aa

FWIW, the system is Ubuntu 20.04 based, and is deployed via kayobe/kolla-ansible.