Chapter 5. Interconnecting OSPF Areas

Terms you'll need to understand:

• Area

• Backbone

• Backbone router (BBR)

• Internal router (IR)

• Area border router (ABR)

• Autonomous system boundary router (ASBR)

• Virtual link

• Router ID

• Loopback interface

• Stub area

• Totally stubby area

• Not-so-stubby area (NSSA)

Techniques you'll need to master:

• Design your area network properly

• Configure virtual links and loopback interfaces

• Master the various link-state advertisement (LSA) types

• Understand how multicasting works for internal routers

• Deploy route summarization in OSPF NBMA environments

• Configure and verify multi-area OSPF operations

Now that you are familiar with the concepts and techniques for operating OSPF in a single-area implementation, it is time to move on to more scalable solutions involving multi-area network designs. Autonomous systems that comprise a single area are often divided into smaller multiple areas for the following reasons:

• A growing network is creating too much overhead on your routers because of the increasing number of SPF recalculations that occur every time a change is made to the topology.

• The link-state topology table is getting too large and unmanageable.

• The routing tables are getting overwhelmed with entries from a large single-area deployment.

• Your network is affected by departmental, geographical, or political changes.

In this chapter, we continue our discussion of OSPF and look at linking multiple areas and OSPF addressing in multiple areas, as well as the different types of areas, routers, and link-state advertisements. Also, you continue configuring and verifying your Cisco router for OSPF multi-area operations.