According to Cisco, OSPF area is a collection of logical OSPF networks (routers and links) that have the same area identification. Depending upon the design of your network, your routers can be part of a single area or multiple areas. Why do we have to complex things and put the router in different areas? Actually, there are many reasons, and one of the many reasons is that it reduces the size of OSPF database when OSPF network is divided into different areas. Think of it this way, dividing the routers into different areas, reduces the size of the database, reduces the frequency of SPF calculation and smaller routing table. Thus, fewer requirements on router memory and CPU. I am not saying that SPF calculation is exhausting the CPU and router memory but the sending and flooding of the new topology information does.

OSPF AREA TYPES

No matter what your OSPF design in your network, all areas you have created should be connected to the backbone area. Backbone area or Area 0 (0.0.0.0) is like the meeting points of all these non-backbone areas. So, it is not just an option or recommendation but a must in every OSPF network design. Although there is an option called virtual link, it is not a good network design but just a remedy to a broken OSPF design.

Now, not all LSA Types are allowed in different areas. Some LSA Types are restricted and depend upon which area type is configured. This further reduces the link state database and routing tables. Injecting too many external routes are also memory intensive. However, all LSA types are allowed in the Normal area. An area which is not configured as Stub, Totally Stub, NSSA, NSSA Totally Stub area is called Standard/Normal Area. We can say that the backbone area is behaving like a Normal /Standard Area as LSA Type 1-5 are not being restricted in this area. When the network is divided into different area numbers (non-zero), an ABR is used to connect the Standard area to the Backbone Area.

On my next post, I will introduce some of the area types which have their own rules in restricting LSA Types.