IPv6 is lost immediately after being assigned
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I've been trying to get IPv6 working and it has been a big task so far. I don't understand what's going on.
Short story, ISP provides dual-stack Internet via PPPoE with public IPv4 IP and DHCPv6-PD for a /56 IPv6 range. My WAN interface is set to "DHCP6" and my LAN (and 2 VLANs) are set to Track Interface. WAN interface also receives it's own /128 IPv6 outside of the /56 delegated pool.
Whenever I boot my device, restart it, reconnect PPPoE, I get everything working for a few seconds, then everything fails. The WAN interface loses it's own /128 IPv6 address, the LAN and VLAN interfaces lose their /64 tracked IPs and all clients stop working properly (because they still have a GUA IPv6 that they will prioritise over IPv4).
Looking at dhcp6c logs it seems that imediately after it gets the proper assigned IPs, dhcp6c restarts and releases all assigned IPs and then tries again but fails.
This is really weird, as 2 times out of 10 of rebooting, restarting PPPoE, it magically starts working. If the IPs are there for more than a few seconds then I'm safe and I have full IPv6 connectivity for as long as the router is up and doesn't restart.