After upgrading from v19.07 to v21.02 on a Linksys WRT1900ACS v2, I reimplemented the configuration, although I can't make IPv6 working.
One problem I spotted is my PC is not using the proper IPv6 link-local address as gateway.
Here is the IPv6 address from the router:
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1508 qdisc mq state UP qlen 1024 link/ether c4:41:1e:30:40:ff brd ff:ff:ff:ff:ff:ff inet6 fe80::c641:1eff:fe30:40ff/64 scope link valid_lft forever preferred_lft forever
My PC get allocated a publicly routable IPv6 through DHCPv6 on its interface talking with the router, which I can ping from the router, but its associated gateway is wrong:
The route table confirms that:
Why are the last 8 bits from the link-local IPv6 from the router interface zeroed in the routing table of the PC? It looks like an odd
/120 representation of the original address.
I rolled back to v19.07: no working IPv6 is a game changer.
That, added to the other problem I encountered.