IPv6 Unicast Topology

IS-IS can support an alternate IPv6 unicast topology, in addition to the normal IPv4 unicast topology, and add the corresponding routes to inet6.0. You can configure the IS-IS interface metrics for the IPv4 topology independently of the IPv6 metrics. You can also selectively disable interfaces from participating in the IPv6 topology while allowing them to continue to participate in the IPv4 topology, or the opposite. You can thus control the paths that unicast IPv4 or unicast IPv6 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 the topology is loaded with reachability information for a certain protocol (for example, IPv6), 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.