Default OSPF Multiarea Setup

MikeO5422

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?

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.