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!
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.