5.3 Using and Configuring OSPF Multi-Area Component
5.3.1 Configuring OSPF ABRs
This section covers some of the OSPF capabilities and associated configurations in more detail. You will learn how to configure an ABR, how to configure route summarization, how to use and configure stub/totally stubby areas, and how to use and configure virtual links.

There are no special commands to make a router an ABR or ASBR. The router takes on this role by virtue of the areas to which it is connected. As a reminder, the basic OSPF configuration steps are as follows:

  1. Enable OSPF on the router.

    router(config)#router ospf process-id

  2. Identify which IP networks on the router are part of the OSPF network. For each network, you must identify to which area the network belongs. When configuring multiple OSPF areas, make sure to associate the correct network addresses with the desired area ID.

    router(config-router)#network address wildcard-mask area area-id

  3. (Optional) If the router has at least one interface connected into a non-OSPF network, perform the proper configuration steps. At this point the router will be acting as an ASBR. How the router exchanges (redistributes) non-OSPF route information with the other OSPF routers is discussed in "Optimizing Routing Update Operation."

Refer to "Configuring OSPF in a Single Area," for details about basic OSPF configuration commands. The main figure shows the configuration commands used to make a regular and an ABR router.

Note The most common OSPF configuration error is to assign the network(s) to the wrong area.