How to Configure Multiarea OSPF

OSPF can be implemented as single-area OSPF or multiarea OSPF. The basic steps for a single area or multiarea OSPF implementation are the following:-

Gather the network requirements and parameters – The first step for implementing multiarea OSPF is the collecting network requirement and important parameters. The requirement includes the number of hosts and required network devices, the IP addressing plan, the routing domain size including, the size of the routing tables. It is also important to consider the risk of topology changes, and other network characteristics.

OSPF parameters – The first step is the base of the second step, the network administrator must find out if single-area or multiarea OSPF is the ideal implementation. If it is multiarea OSPF, there are several considerations to determine the OSPF parameters including IP addressing plan, OSPF areas, and Network topology.

Configure multiarea OSPF – Configure the multiarea OSPF implementation based on the parameters.

Verifies the Multiarea OSPF – Verify the multiarea OSPF implementation based on the parameters.

Configuring Multiarea OSPFv2

Figure 1 displays the reference multiarea OSPF topology. In this example:

• R1 is an internal router for area 10 because all of its interfaces are in area 1.
• R2 is an ABR because it has interfaces in area 10 and also an interface in area 0.
• R3 is also an ABR because it has interfaces in area 20 and an interface in area 0.
• R4 is an internal router for area 20 because all of its interfaces are in area 20.

There are no unique commands for multiarea OSPF configuration. A router simply becomes an Area Border Router (ABR) when it has two “network” statements in different areas.

Both network 192.168.1.0/24 and 10.10.10.8/30 are configured in the same area because this is an internal router for area 10. The autonomous number for the OSPF is 10. Now let’s configure the R2, which is the ABR for Area 10 and Area 0 (backbone area). The configuration of R2 is the following:

The above configuration enables OSPF in the two different areas. The GigabitEthernet 0/2 interface is configured as part of OSPF area 10 and GigabitEthernet 0/0 as part of OSPF area 0. Because R2 has interfaces connected to two different areas, so it is an ABR between area 10 and area 0. Now I am going to configure R3, which is also an ABR. The configuration is:

You can see the configuration of R3, GigbitEthernet0/0 is the part of area 0 and GigabitEthernet 0/2 is the member of area 20. The R2 is a member of both area 0 and area 20, therefore it is an ABR for both areas.  R4 is the internal router of area 20, the router-id of the R4 is 4.1.1.1. the configuration is simple, just like R1:

Configuring Multiarea OSPFv3

The OSPFv3 is not more different than the OSPFv2, we are using the same topology for multiarea OSPFv3 configuration. The figure-2 illustrates the topology with the IP addressing scheme. A router simply becomes an ABR when it has two interfaces in different areas just like OSPFv2 and there are no special commands required.

Now lest come first of all configure the R1, which is the member of area 10 because all interfaces are connected to area 10. We should assign the same router IDs as OSPFv2.