What is OSPF Stub Area?

Yasir Irfan Profile: Yasir Irfan

We all know OSPF as routing protocol is one of most widely used IGP protocol, OSPF happens to be the most scalable IGP protocol. OSPF also happens to be one of complex protocols as it deals with various concepts and terminologies. Once such topic where people get confused is OSPF Area types.

I will try to simplify them and present them in an easy language, I am not going to reinvent the wheel , as one can find plenty of resources for OSPF.

What is a OSPF Stub Area ?

OSPF Stub Area basically filters out an information of a OSPF database purely based on the LSA types, Basically  an ABR  in a Stub Area  prevents LSA type 5 to be flooded in to a Stub Area , it removes Type 5 LSA  and replaces them with a default route which is a Type 3 LSA.  To simplify  ABR creates a default route using LSA 3 , listing a  0.0.0.0 with a subnet mask  0.0.0.0 and flood the same into stub area. By using Stub Area feature one can reduce the CPU utilization of a Router .

From the above scenario we can see Type 3 LSA is exchanged between Area 0 and 5 , however when a Type 5 LSA reaches R2 which is an ABR , it will strip External LSAs  (Type 5 LSAs) and replace them with default route towards the Router R3.

OSPF Stub Area is configured in Cisco Routers using an IOS command

router ospf 1

area 5 stub

In the upcoming post lets see how to configure OSPF stub area in Cisco Routers, we will build a sample topology using Cisco VIRL.

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