Understanding IS-IS IPv4 and IPv6 Unicast Topologies
You can configure IS-IS to calculate an alternate IPv6 unicast topology, in addition to the normal IPv4 unicast topology, and add the corresponding routes to inet6.0. The IS-IS interface metrics for the IPv4 topology can be configured independently of the IPv6 metrics. You can also selectively disable interfaces from participating in the IPv6 topology while continuing to participate in the IPv4 topology. This enables you to exercise control over the paths that unicast data takes through a network.
A topology is the set of joined nodes. IS-IS evaluates all the paths in a single topology for each IS-IS level and uses the shortest-path-first (SPF) algorithm to determine the best path among all the feasible paths. Topology discovery and SPF calculation is performed in a protocol-neutral fashion because it is done at Layer 2 of the OSI model. If you load the topology with reachability information for a certain protocol (for example, IP), the assumption is that the circuits that are supposed to provide reachability between routing devices can carry the protocol. The SPF algorithm has a per-link orientation, not a per-address family or per-protocol orientation.
Multitopology routing enables you to override this default behavior by enabling a per-address family, per-protocol SPF calculation.
The additional CPU load associated with multiple runs of the SPF algorithm is generally not an issue with the processing power available on today’s routing device control planes.
The multitopology extensions alter existing type, length, and value (TLV) tuples by adding a topology ID. Each routing device in a given topology maintains its adjacencies and runs a per-topology SPF calculation.