Configure multiarea OSPF

OSPF uses areas to simplify administration and optimize traffic and resource utilization. An area is simply a logical grouping of contiguous networks and routers. All routers in the same area have the same topology table and don’t know about routers in the other areas. In this lesson we will describe how you can configure a multiarea OSPF.

Consider the following multiarea OSPF network:

In the network we have three routers and two hosts. We need to configure OSPF with routers in two areas – area 0 and 1. R2 should connect two areas, which will make him an ABR (Area Border Router).

Here is the OSPF configuration on R1:

Configuration of R3 looks similar, but with the difference in area number, since R3 is in area 1:

What about R2? Because R2 is an ABR, we need to establish neighbor relationship with both R1 and R3. To do that, we need to specify different area ID for each neighbor relationship, 0 for R1 and 1 for R2. We can do that using the following commands:

R2 should establish a neighbor relationship with both R1 and R3. We can verify that by using the show ip ospf neighbor command on R2:

To verify if directly connected subnets are really advertised into the different area, we can use the show ip route ospf command on both R1 and R3:

The characters IA in front of the routes indicate that these routes reside in a different area.