| 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.
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