Mismatch of OSPF Cost

Answered Question
Aug 11th, 2010

Hi All,

I would like to clarify what happens to traffic flow when there are mismatched OSPF link costs (see topology diagram attached).

On the diagram, traffic from Network A to Network B will first hit R1, I assume that as the OSPF interface cost to R2 is lower than R3, traffic is then forward to R2 > R4 > Network B.

On the return path back to Network A I am trying to establish what costs would R4 have in it's database? and what path it would take?

Is cost applied on ingress/egress? how is it advertised within the area?

Your assistance is appreciated!

Many Thanks

Kris

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Correct Answer by Jon Marshall about 6 years 3 months ago

Kris

Correct. In this case you have asymmetric routing because from A -> B the path is R1-> R2 -> R4 and from B -> A. as you say, the path is R4 -> R3 -> R1.

Jon

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Jon Marshall Wed, 08/11/2010 - 08:25

Kris

The OSPF cost is carried in the LSAs that are exchanged within an OSPF area. When a  router calculates the cost to a destination it uses the cost of the exit interface of each router in the path to the destination. So obviously the path one way may not be the same as the return path and can lead to asymmetric routing.

Jon

kris-andrews Wed, 08/11/2010 - 08:39

Thanks for your response,

So if OSPF calculates cost based on exit interfaces to the destination, going back to the diagram, so from Network B to Network A the traffic flow would be R4 > R3 > R1 with a cost of 20 - is that correct? as the the path through R2 would have a cost of 40.

Kris

Correct Answer
Jon Marshall Wed, 08/11/2010 - 08:42

Kris

Correct. In this case you have asymmetric routing because from A -> B the path is R1-> R2 -> R4 and from B -> A. as you say, the path is R4 -> R3 -> R1.

Jon

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