Default OSPF Multiarea Setup

Quick question, I believe I have the answer I am just looking for support.

If I have three routers, with interfaces as follows:

R1:
fa0/1 - 192.168.1.1 255.255.255.0 LAN
fa0/2 - 192.168.2.1 255.255.255.252 (connects to R2)

R2:
fa0/1 - 192.168.2.2 255.255.255.252 (connects to R1)
fa0/2 - 10.0.2.2 255.255.255.252 (connects to R3)

R3:
fa0/1 - 10.0.1.1 255.255.255.0 LAN
fa0/2 - 10.0.2.1 255.255.255.252 (connects to R2)

Now I enable basic multiarea OSPF
R1:
network 192.168.0.0 0.0.0.255 area 0

R3: (ABR)
network 192.168.2.0 0.0.0.255 area 0
network 10.0.0.0 0.255.255.255 area 2

R3:
network 10.0.0.0 0.255.255.255 area 2

There technically is zero benefit to enabling multiple areas as the type 1 and type 2 LSAs will not be summarized into each area. As a result, the same amount of LSAs will exist in the database. (as type 3 LSAs instead of type 1 and 2) If there is a change, the same amount of LSA traffic will be generated/flooded and the same amount of OSPF processing (okay maybe technically not the EXACT same but pretty close to it) will be performed, just on type 3 LSAs instead of type 1 and 2 LSAs.

Is this an accurate statement? If one enables multiarea OSPF with no summarization are there any benefits?

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Comments

fredrikjj

It's not accurate. The short answer is that each area has its own link state database and changes to links within an area prompts a recalculation of the shortest path tree within that area. However, a recalculation (often called an "spf run") is only done for the area where the topology change happened.

MikeO5422

Thanks for the response.

Just a quick follow-up - if you are in area 0 and a summary LSA originating from an ABR in area 1 gets removed from the area 0 LSDB, SPF is not run in area 0?

MikeO5422

Finally got a chance to run this in a simple lab. Indeed no SPF calculations for the removal of type 3 summary LSAs in other areas. Not sure where I got the idea that they would cause SPF to run, but its not correct.