Comment 1 for bug 1953165

So this happens when you have more than one DHCP agent only, correct? Using isolated subnets?

It looks like an oversight when we added support for metadata over IPv6, since using the same link-local address on multiple nodes will fail in DAD as you show above.

Just thinking out loud there might be a couple of options:

1) Neutron tells only one DHCP agent to configure the IPv6 metadata address. It reduces availability, and there might be some edge cases, but could work.

2) We change to use an Anycast address, in which only one of the nodes will get the request. But this is more complicated as 1) Anycast addresses are only supposed to be configured on routers (which don't exist here); and 2) IANA assigns Anycast addresses, https://www.iana.org/assignments/ipv6-anycast-addresses/ipv6-anycast-addresses.xhtml

A quick fix for you would be to set this in neutron.conf:

dhcp_agents_per_network = 1