TCP/IP Guide

OSPF Hierarchical Topology

When the number of routers in an AS is relatively small, using the previously described basic topology works well. Each router maintains a common picture of the network topology in the form of an identical LSDB. The routers communicate as peers using LSAs. While changes in the AS may cause a router to temporarily have different information than its peers, routine exchanges of data will keep all the LSDBs synchronized and up-to-date, and not that much information needs to be sent around because the AS is small.

This simpler topology does scale reasonably well, and it can support many smaller and even moderate-sized ASes. However, as the number of routers increases, the amount of communication required to update LSDBs increases ...