IPv6-Only Links and Multi-Topology IS-IS Mismatch
A dual-stack network with five SR OS routers uses IS-IS level 2 for both IPv4 and IPv6 in single-topology mode. Two new aggregation links are IPv6-only because the IPv4 addressing plan is exhausted; the links are configured with IPv6 addresses and IS-IS, and adjacencies come up. IPv4 traffic works everywhere else. IPv6 pings between routers on opposite sides of the new links fail, even though show router isis adjacency shows Up and the IPv6 interfaces are pingable locally. The design team enables multi-topology on three of the five routers. After that, IPv6 routes appear on some routers but not on the two routers connected to the IPv6-only links. One router logs 'MT topology not supported'. The provider wants IPv6 to use the new links while keeping IPv4 on the existing topology.
1. Why can the IPv6-only links form an IS-IS adjacency but still not carry IPv6 traffic in single-topology mode?
Read the worked answer
In single-topology IS-IS, IPv6 is not a separate topology. The IPv4 topology is used to compute both IPv4 and IPv6 paths, and IPv6 reachability relies on IPv4 next-hop addresses on the interfaces. An IPv6-only link has no IPv4 address, so it is not part of the IPv4 topology. The adjacency can still come Up because IS-IS runs directly over layer 2 and does not require an IPv4 address on the interface for hello exchange. However, the link is absent from the IPv4 SPF tree, so IPv6 prefixes advertised over that link have no usable path. Local IPv6 pings may work because they are link-local or directly connected, but transit IPv6 traffic fails. The symptom is therefore an Up adjacency with missing IPv6 routes, not a down adjacency. Multi-topology is the correct design because it gives IPv6 its own SPF tree and allows links that have no IPv4 address to be used for IPv6.
2. Explain why enabling multi-topology on only three of the five routers produces partial IPv6 reachability, and what the 'MT topology not supported' log indicates.
Read the worked answer
Multi-topology requires consistent support across the level. MT-capable routers advertise MT capability and MT reachability TLVs, including a topology identifier; IPv6 uses MT ID 2. A router that is not MT-capable ignores those TLVs and continues to use single-topology behavior. The MT-capable routers may build a separate IPv6 topology among themselves, but they cannot include non-MT routers or links that are only advertised by non-MT routers. The result is an inconsistent IPv6 graph: some routers have IPv6 routes through the MT-capable subset, while the two routers on the IPv6-only links do not. The log 'MT topology not supported' indicates that a router received MT TLVs from a neighbor but is not configured to process that topology, or that the neighbor advertised a topology the router cannot support. The fix is not to enable MT on more interfaces selectively; it is to enable MT for IPv6 on all routers in the level that must participate in IPv6, and to verify that every router supports the same MT ID.
3. Give the SR OS configuration and verification steps to complete the migration so that IPv6 uses the new links and IPv4 remains stable.
Read the worked answer
Enable multi-topology for IPv6 under the IS-IS instance on all five routers, not just three. On SR OS this is done in the IS-IS configuration context with the multi-topology command; confirm the exact keyword for the release, and ensure IPv6 routing is enabled on the interfaces. Keep IPv4 in the existing topology and let IPv6 use MT ID 2. After enabling MT on the remaining routers, verify that every router's LSP contains the MT capability TLV and the MT IPv6 reachability TLVs. Use show router isis database detail on the routers at both ends of the IPv6-only links and confirm that the IPv6 prefixes are present with the correct MT ID. Use show router isis adjacency detail to confirm that all adjacencies are still Up, and show router route-table ipv6 plus show router isis routes ipv6 to confirm that IPv6 routes point over the new links. Test IPv6 ping and traceroute across the new links. Finally, verify that IPv4 routes and adjacencies are unchanged, because a common MT migration error is to disturb the IPv4 topology while adding IPv6.