Case 1 --two default routers--

interface rtk0 down

(1)At first

Route of going and return decide at random. When going pass trough Power2, return can pass through Power3. Now, at first Dyna1 choose Power3 as the default router. Packet of "ping6" go trough Power3. And echo reply return trough Power3.

routing tables

Dyna1
Power1
Power2
Power3

ndp entry at Dyna1

Dyna1

(2)Interface rtk0 at Power3 down

Dyna1 don't change route, and sent packet to Power3. Power3 continue to have routing table to go [2001:200:08410::/64].

(3)Interface rtk0 at Power3 down

So, I delete passive mode in wm0 for changing route of OSPF. Power2 and Power3 can know a mutual state by doing so. So, Power3 became to know Power2 as a better route. And Power3 send ICMP redirect to Dyna1. Dyna1 reseave it, and send same data to Power2. After that it became possible to connect as the picture below.