ABSTRACT
This chapter introduces the hierarchical structures of Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS) and shows that distance vector routing applies to the case of hierarchical networks. It discusses the Link State Database (LSDB) structure and distance vectors of OSPF and IS-IS and describes several restrictions that may lead to non-optimal path selection in hierarchical networks. The structures of OSPF and IS-IS hierarchical networks are usually considered very different. This idea seems to be rooted on the way OSPF and IS-IS routers handle the Area IDs: in OSPF the Area identifiers are per interface and in IS-IS they are per router. The support of hierarchical routing in OSPF and IS-IS may require the addition of new elements to the LSDB. As in OSPF, IS-IS keeps the Type-Length-Value of single-area networks to describe the routing information internal to an area.
