Interconnecting OSPF Areas

Interconnecting
OSPF Areas
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:


Understand how OSPF operates in multiple areas.


Understand how to configure OSPF in multiple areas.


Identify the steps to verify OSPF operation in multiple areas.


When route selection and configuration problems occur,
understand how to use the various show and debug
commands to determine the cause of the problem.

In 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 also identifies the various categories of routers used in multi-area configurations.
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 BSCI exam, and we will detail the types of LSAs used by OSPF. We will show 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
boundary 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 will conclude with a collection of
debug
and
show
commands
that can be used to effectively monitor and troubleshoot a multi-area OSPF implementation.