Configuration: FB4040 with a freshly installed OpenWRT 19.07.2 behind a FB6490 used for cable access (dual stack lite).
PC --- LAN --- FB4040 --- WAN/WAN6 --- FB6490 --- ISP
The FB4040 ist configured fairly close to the OpenWRT default setup and IPv4 is working fine. IPv6 isn't though. The WAN6 interface is setup as DHCP6 client and appears to correctly receive a delegated network for its LAN. A PC within the LAN can register with DHCP6 and receives an IPv6 within this delegated network. So far everything looks nice.
IPv6 internet pings by the FB4040 are successful, using its WAN6 side address. Pings from the PC in the LAN local to the FB4040 however are not. A tcpdump on the WAN6 interface of the FB4040 shows the packets passing outbound, but there is not reply. The 6490 does not have a trace option, so I cannot see whats going on there.
A wireshark trace within the WAN/WAN6 network between the FB4040 and the FB6490 shows router advertisements for 2xxx:xxxx:xxxx:2000::/56 by the outer FB6490. It does not show any router advertisements by the inner FB4040 though.
I'm not perfect with the IPv6 stuff, but expected the inner FB4040 to place it's own router advertisements for 2xxx:xxxx:xxxx:20fc::/62 on its exterior network, else the stations there, including the outer FB6490, do not know where to route the LAN addresses..
Still, the only router advertisements shown on the intermediate network between both routers are those of the outer router every 10 minutes. The inner router, the OpenWRT, advertises in 5 minutes intervals on its niside network (LAN) only, but not on its outside WAN6.
Thanks. Took a while but I got it in the meantime too.
host2 => host1 fails. Wireshark on host1 sees the packet, but there is no reply. On host1, there is no specific route to the LAN shown. It however appears to have 2 default routes, one for each router, and additionally a /56 route to FB6490.
The first case fails because you have asymmetric routing. Echo request goes host2->fb4040->host1. Reply goes host1->fb6490 where it is blocked as invalid, because the firewall on 6490 never examined the initial packet. You need to find in the config of 6490 how to disable this.
The third case is weird, but still it is not an issue of OpenWrt, if you can see the packet in tcpdump leaving wan6 with correct source and destination IPs.