OSPF configuration

Configuring OSPF 1

OSPF basic are configuration is very simples. Just like with other routings protocol covered so far (RIP and EIGRP) first you need to the enable OSPF on router. This is done by using router OSPF PROCESS-ID global configurations commands. Next you need to the define on which interfaces OSPF will be run and what networks will have advertised. This is done by using network IP_ADDRESS WILDCARD_MASK AREA_ID command from the OSPF configuration modes. 

Let’s get started with the some basic OSPF configurations. We will be use the following networks topology:

First, we need to the enables OSPF on both routers. Then we need to the defines what network will have advertised into OSPF. This can have done by using the followings sequence of the commands on both router:

The networks command entered on the both routers includes subnets directly connected to the both routers. We can verify that routers have become neighbours by typing show internet protocol OSPF neighbours command on either router:

To verify, the routing updated were are exchange. we can use show internet protocol route commands. All routes are marked with character O are OSPF routes. For the example, here is output of command on R1:

You can see that R1 learned about network 192.168.0.0/24 through OSPF.

Configuring OSPF 2

Although basic are OSPF configuration can be very simple, OSPF provides are many extra feature that can get are really complex. In this example, we will configure are multi area OSPF network and some other OSPF feature.

Consider the following multi area OSPF network:

Let’s take examples, we have two OSPF areas - area 0 and area 1. you can see from network topology depicted above routers R1 and R3 are in area 0 and area 1 respectively. Router 2 connects to the both areas which make him a ABR (Area Border Router). Our goal is to be advertise subnets are directly connect with R1 and R3. To do that, the following are configuration on R1 will be used:

Because R1 connects to R2 only, we only need to be establish a neighbour relationships with the R2 and advertise directly connected subnet into the OSPF.

Configuration of R3 looks similar, but with the one differences namely area number R3 in the area 1

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

now R2 should have neighbour relationships with both R1 and R3. We can verify that by using show ip ospf neighbour command:

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

Character IA in front of routes indicate that these are routes reside in differents areas.