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 [...]

Advertising a default route in OSPFv3 is somewhat similar to IPv4 OSPF.
Without using the “always” command. First we need to set a static default route. Here we set it to Null0.
# ipv6 route ::/0 Null0
Then we advertise the default route.
# ipv6 router ospf 1
# default-information originate
With the “always” command you do not need a default [...]