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n this chapter, we will illustrate the scalability constraints of an
OSPF network with a single area. The concept of multi-area OSPF will be
introduced as a solution to these scalability limitations. This chapter will
also identify the various categories of routers used in multi-area configura-
tions. These router categories include a backbone router, internal router,
area border router (ABR), and autonomous system boundary router (ASBR).
We'll explore how these routers can use summarization and default routes to
reduce the amount of route information that is injected into an area, thus
reducing a router's memory and processor overhead.
The functions of different OSPF Link State Advertisements (LSAs) are
very important to understand for the Routing exam, and we will detail the
types of LSAs used by OSPF. We will see how these LSAs can be minimized
through the effective implementation of specific OSPF area types.
Specifically, we will examine stub areas, totally stubby areas, and not-so-
stubby areas and show how these areas can be used to minimize the number
of LSAs advertised into an area. We'll also provide a set of design guidelines
and configuration examples as well as the syntax required to configure route
summarization at both area border routers and autonomous system bound-
ary routers.
You'll learn that all areas need to have a link to Area 0. If an area is not
attached to Area 0, virtual links can be used to span transit areas in OSPF
networks where all areas are not physically adjacent to the backbone area.
We then will conclude with a collection of debug and show commands that
can be used to effectively monitor and troubleshoot a multi-area OSPF
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