With one of our network providers our Linux based router (Vyatta/Debian) can't talk to their Cisco router over IPv6. Their router seems to not reply to the ICMP6 neighbor solicitation requests that ours send. This is working fine between our two routers on this VLAN and between our routers and every other provider and peer we have; IPv4 on the same interface to the same provider is working fine, too.
The interface configuration on their router (IP addresses munged) is:
interface GigabitEthernet1/48.301 encapsulation dot1Q 301 ip address 10.0.0.1 255.255.255.248 no ip redirects no ip proxy-arp ip mtu 1500 ipv6 address 2078:abcd::5b::1/112 ipv6 mtu 1500 no ipv6 redirects no cdp enable end
Which all looks pretty default/standard to me. When I got them to ping our side of the interface, a ping in both directions immediately started working. (Our router replied to the ND request and I guess our router figured out their mac address from the ICMP6 ECHO request).
If we add an entry to the neighbor table manually, everything is working fine, but obviously that's a bit of a hacky solution.