Routing from Level 1 Domains into the Level 2 Domain
As we discussed earlier, information hiding is one of the keys to building a truly large, scalable network. IS-IS hides information to reduce the flooding scope and database sizes in individual intermediate systems by allowing the network designer to break up the network into domains. Let's look at the interaction between level 1 and level 2 routing.
An L1/L2 intermediate system will not inject destinations learned from its level 2 adjacencies into a level 1 domain; in other words, it will not advertise destinations reachable through one of its level 2 neighbors to any of its level 1 neighbors. Figure 3-4 illustrates how this works.