OSPF Area, OSPF Layered Network Design, and advantages of OSPF Areas

Open Shortest Path First (OSPF) is the recommended IGP for very large enterprise networks. Entire Open Shortest Path First (OSPF) network can be be divided up into small networks called OSPF Areas. Open Shortest Path First (OSPF) supports hierarchical network design. Open Shortest Path First (OSPF) allows the network to be designed in two layer hierarchies. Area 0 at one layer and all other Areas at other layer.

An Open Shortest Path First (OSPF) Area must be configured as a group of contiguous IP networks. This allows Route Summerization at Area level.

An OSPF Area is a collection of OSPF routers and networks that share the same Link State Database. A Router in one OSPF Area doesn’t have detailed information about network topology outside of its area.

Advantages of designing Multi-Area OSPF networks are listed below.

The Routers in same OSPF Area share the same Link State Database (LSDB). The memory and processor requirement for Link State Database (LSDB) in Routers are less.

Whenever a network topology change happens in a network, the Routers need to re-run the SPF algorithm to calculate best routes. The SPF algorithm is required to run when the topology change happens in the same area. Hence OSPF Routers within Area have to run SPF less often.

Manual Route summarization can configured only on ABR and ASBR, which allows the Areas to exchange summerized routing tables between each other

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