ABSTRACT
This chapter presents several experiments that illustrate the various features of Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS) hierarchical networks. Both OSPF and IS-IS networks can be structured in a two-level hierarchy of areas, where the upper-level can only have one area and the lower-level areas attach directly to the upper one and cannot attach to each other. The chapter explains the router configurations and the Link State Database (LSDB) structure. It shows how the restrictions imposed on the shortest path selection process can lead to non-optimal routing. The router configurations of OSPF are very similar to the single-area ones. The advertisement of a domain-external prefix in OSPF hierarchical networks requires two Link State Advertisement types, one to advertise the prefix and another to advertise the Autonomous System Border Router that injected the prefix into the domain. The chapter illustrates the consequences of restricting the shortest path selection process.
