IPv6 IS-IS supports Multi-Topology (MT) mode, which allows you to configure both IPv4 and IPv6 topologies on the router interfaces in an area or a domain. However, when implementing an MT, all routers in an area (Level 1 routing) or a domain (Level 2 routing) can be configured with a set of independent topologies on all their interfaces, even on loopback interfaces. All routers in an area or a domain use the same type of IPv6 support, either single-topology or MT. In a network, the Shortest Path First (SPF) is calculated for each configured topology.
The figure below depicts a non-congruent topology with IPv6 IS-IS MT enabled. Router 1 is an IPv4 and IPv6 dual stack router, Router 2 is an IPv6 router, and Router 3 is an IPv4 router. All the routers (Router 1, Router 2, and Router 3) in the Area 1 are configured with a set of independent topologies.
