Question

A service provider runs IS-IS on all IOS XR core routers for both IPv4 and IPv6. Historically the network used a single topology (default) for both address families. An engineer enables 'address-family ipv6 unicast' under a new router's IS-IS process and configures 'multi-topology' on that router only. After the change, IPv6 routes learned via IS-IS from that new router are missing on adjacent routers, although IPv4 routing and the IS-IS adjacency itself remain fully up. What is the most likely cause?

Accepted Answer

The adjacent routers use single-topology (single SPF) IS-IS, so they advertise IPv6 reachability using the standard IPv4 TLVs, and the multi-topology router only processes MT TLVs — the topology mode mismatch prevents IPv6 prefix exchange.

Answer

Enabling multi-topology automatically disables the IPv4 address family, so the adjacency is only exchanging IPv6 in a separate topology that the neighbors cannot see.

Answer

IS-IS cannot carry IPv6 prefixes unless wide metrics (metric-style wide) are disabled globally on all routers in the area.

Answer

The new router formed an adjacency in a different IS-IS level, so IPv6 prefixes are being leaked incorrectly between Level-1 and Level-2.