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

Next, I do Interface of Power3 down(rtk0). Dyna1 choose Power2 as the default router. And "ping6" can reach. But echo reply cannot reach Dyna1. It is because routing table to go prefix 2001:200:0:8411::/64 in Power1 has not change yet. So, echo reply is not reach to Dyna1.

(3)Interface rtk0 at Power3 down

Power1 change the routing table to go prefix 2001:200:0:8411::/64. "ping6" can go to "gr2000.k2c.wide.ad.jp" and can return.

routing tables

Dyna1
Power1
Power2
Power3

ndp entry at Dyna1

Dyna1

I'm not happy because it has no problem.