08-11-2010 07:27 AM - edited 03-06-2019 12:25 PM
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
Solved! Go to Solution.
08-11-2010 08:42 AM
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
08-11-2010 08:25 AM
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
08-11-2010 08:39 AM
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
08-11-2010 08:42 AM
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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