Overview
As explained in the previous chapter, OSPF relies on complex communications and relationships to maintain a comprehensive link-state database. However, as an OSPF network scales to 100, 500, or even 1000 routers, link-state databases can balloon to include thousands of links. To help OSPF routers route more efficiently and to preserve their CPU and memory resources for the business of switching packets, network engineers divide OSPF networks into multiple areas.

Because it has the capability to break a large network into small manageable units, OSPF enjoys tremendous scalability, but that scalability comes at price. Multiarea OSPF networks can be difficult to design, and typically demand more administrative attention than any other popular interior gateway protocol.

This chapter describes how to create and configure OSPF areas. Specifically, this chapter examines the different OSPF area types, which include stubby, totally stubby, and not-so-stubby areas (NSSAs). Each of these areas use special advertisements to exchange routing information with the rest of the OSPF network, so you will study link-state advertisements (LSAs) in detail. You will also look at the Area 0 backbone rule and how virtual links can work around backbone connectivity problems. Finally, this chapter surveys important show commands that can be used to verify multiarea OSPF operation.