With the addition of a couple of TLVs and some minor changes to the way SPF is applied to the available reachability information, IS-IS can run over multiple overlaying topologies, treating each one as if it were a separate network. For instance, in Figure 6-14, the network designer may want intermediate systems A through E to be available for normal IP traffic, but A, B, and E to be available only for traffic traveling to and from a particular site, or to provide a specific class of service.
Figure 6-14. A network with multiple topologies
The primary packet change is the addition of a multi-topology identifier TLV to be ...