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I am studying OSPF, and It seems to me that totally stub areas just saves the day. Why do we need stub areas?

To be more clear, as I read, I understand that OSPF areas designed as star topology, so there is one backbone area, and the other areas. So other areas connected to backbone area thru an ABR.

When I define that area as stub, how are the type 3 LSAs coming from backbone used, because there is only one connection to backbone area (a complete default route is enough indeed)?. In other words, can you give me a sample topology, that I should define the area as stub, but not totally stub?

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You can have multiple ABRs which connect an area to Area 0. With a stub area, a router in the area could choose the best ABR in the path to the destination, but with a totally stubby area, it will just choose the nearest ABR.

For instance, if you have two ABRs for your area (Area 1), each ABR will have, at least Area 0 and Area 1 interfaces, but one of them could also have an Area 2 interface, or it could be directly connected to an Area 2 ABR with a low cost, but the other ABR would have a higher cost to get to Area 2. It would make sense, when sending traffic to Area 2, to send it to the ABR with the lowest cost to Area 2, but a totally stubby area doesn't know how to do that.

  • I really need to learn how to type faster ;-p – Ron Trunk Mar 14 '16 at 12:26
  • Ok, thanks, I missed the point that, more than one ABR can exist between a non-backbone area and the backbone area. After taking this into account, it seem reasonable now. But for one ABR, I think my statement is right, right? – Salihm Mar 14 '16 at 12:36
  • For one ABR, you normally would use a totally stubby area, unless you need to have external routes come into the area. Then you use a not so stubby area (NSSA). Also, and area which supports a transit link cannot be a stub area. – Ron Maupin Mar 14 '16 at 12:39
  • Thats clear now. All the sample network topologies in the documents were kind of one-abr-between-areas. So I was misleaded by the topology images :).. – Salihm Mar 14 '16 at 12:47

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