The OpenWrt router receives a delegated prefix using DHCPv6, which provides global addresses to the network connected to the OpenWrt router. This works perfectly fine, until the ISP router is restarted, which sometimes happens when it upgrades its firmware. In this situation, the IPv6 connectivity is lost and is not automatically restored. To get IPv6 connectivity, I have to restart the WAN6 interface, which estabilshes the IPv6 connection again. The IPv4 connectivity is restored automatically.
After the IPS router has been restarted, the OpenWrt router cannot ping IPv6 hosts via global addresses. Even the global address of the ISP router cannot be reached via ping. Pings via link-local addresses work fine.
When the IPv6 connection is gone, restarting the WAN6 interface, seems to change nothing on the side of the OpenWrt router. The output before and after the restart of //ip -6 addr//, //ip -6 route//, and //ip6tables -L// are identical (except for the IP address life-times). Also the information displayed by LuCI on the Interface page stays the same. However, the connectivity is restored after the interface is restarted.
My guess for explaining this behavior is as follows. When the ISP router is restarted, it receives the same IPv6 address lease and prefix from my ISP as earlier. (The IPv6 address lease is valid for a few days.) The ISP router, however, has "forgotten" that it had delegated a prefix to the OpenWrt router. Hence, the ISP router is not routing any traffic to the prefix the OpenWrt router is using. Since the IPv6 address of the ISP router does not change, the OpenWrt router does not detect this change. The RAs that the OpenWrt router receives are the same as before. When I restart the WAN6 interface, however, it asks for a new IPv6 address and a new IPv6 prefix, which in turn leads to the ISP router creating the proper route for the delegated prefix, which restores the IPv6 connectivity.
Unfortunately, I do not have access to the routing table of the ISP router, and hence, I cannot verify my hypothesis. (The reason for using OpenWrt is, that the ISP router is severely limited. I have to use the ISP router, however.)
Is there a way to force odhcp6c to probe whether the current delegated prefix is valid? Maybe sending some sort of renew/refresh message via DHCPv6? Is there a way to ask odhcp6c to regularly update/renew the delegated prefix, say every 5 minutes?
I currently have a cron job, which pings an IPv6 host on the internet and restarts the WAN6 interface when the ping fails. This workaround does the trick, but is rather ugly.
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You can force a DHCPv6 renew by sending the SIG_USR1 signal to the odhcp6c process; this could be used to check if the ISP router will reply on the renew.
But this is a workaround for a short coming in the ISP router as the normal way is to support DHCPv6 reconfigure accept in the upstream router to trigger a DHCPv6 renew.