I ran into a weird problem the other day where my IPv6 OSPF adjacency kept flapping every 40 sec(dead timer) and then reestablishing.

Before you go crazy trying to troubleshoot this problem, check this:

1. Check to see if there is a switch in between your IPv6 routers, make sure this switch supports IPv6! if there is a switch in between, try disabling mld snooping or ipv6 routing all together and see if it fixes the problem.

2. Setup the interfaces as non-broadcast and do neighbor statements to the link-local bypassing the Ipv6 multicast for OSPF.

3. If you changed the IPv6 OSPF router-id. Make sure you remove the OSPF process totally, clearing may not help. Then clear out the neighbor OSPF process so that neighbors accept new Router-id and do not have stale information. Check this by doing # show ipv6 ospf database

This is what I had to do to maintain stability.

Cheers!