How to configure OSPF Stub Area

Let me use another topology to explain where/why you could use stub areas:

bgp-ospf-rip-routing

OSPF is our “campus” network. Area 0 is the main network, area 1,2, and 3 are branch offices. BGP routes are redistributed into OSPF, RIP routes are also redistributed into OSPF.

With regular areas, all routers will learn about all prefixes out there. When you look at this picture…why would area 1, 2, or 3 need to know the prefixes from BGP? There is only one path, and that is through area 0. We can make area 1,2 and 3 stub areas…a default route is more than enough.

Also, let’s say a router in area 1 wants to go to a router in area 2. Do we need to know a specific prefix? Not really, there is only one path through area 0…in other words, why not make area 1,2, and 3 totally stub areas…get rid of all inter-area routes and just use a default route.

That does introduce one problem…a router in area 3 is doing redistribution from RIP into OSPF, that’s something we can’t do in a stub or totally stub area. That’s why you should convert area 3 into a totally NSSA…it’s a totally stub area that does allow an ASBR.

Btw, as soon as you configure a router to redistribute something into OSPF, it’s an ASBR.

Hope this helps.

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