Because of hierarchical design the OSPF are more efficient and scalable. The OSPF segments network into different areas. The area is a group of routers sharing the same link-state information in their Link-State Databases (LSDBs). We can implement the OSPF on the following two ways.
- Single-Area OSPF
- Multi area OSPF
Single Area OSPF (Area 0)
Area 0 is also known as the backbone area for OSPF which links all other smaller areas within the hierarchy. The single area OSPF is useful in smaller networks where only few routers are working. The figure below illustrates the single area OSPF network.
Multi Area OSPF
Figure below illustrates the multi area OSPF network. The network consists of multiple OSPF areas in a hierarchal design. All areas has connected to area 0 (backbone area). The interconnecting router between backbone area and other areas known as area border router (ABR).
As shown the figure the multi-area OSPF segments a one large network into several networks. One autonomous system ( AS) has divided into multiple areas to support hierarchical routing. The routing occurs between the multiple areas. The more operation required system resources’ such as re-calculating the database, has done only within a relevant area. For example, when there are changes occurs in the topology like addition, deletion or modification of the link, the router must rerun the SPD algorithm to create new SPF tree and update the routing table in the same area where changes occurs. But changes has share to routers in other areas in a distance vector format to update their routing tables and these router and areas has no need to rerun the SPF algorithm.
Advantages of Multi Area OSPF
Small Link-State Data Base. Arranging routers into areas efficiently partitions a potentially large database into smaller and more manageable databases.
Smaller routing tables – smaller number of routing table entries due to network address summarization between areas.
Reduced link-state update overhead – The reduced link-state processes due to segmentation minimizes processing and memory requirements.
Reduced frequency of SPF calculations – The SPF algorithm is only run in the area where change has occurs. For example, it minimizes routing update impact because LSA flooding stops at the area boundary.