OSPF cost calculation

[ashvanth kumar]

In the above set up if i give sh ip route for 10.242.0.0 in router A, I get the below output where the metric is 5. Any clue how its 5?

COGINCHNTCOCORE#sh ip route 10.242.0.0

Routing entry for 10.242.0.0/16

  Known via "ospf 10", distance 110, metric 5, type inter area

        Route metric is 5, traffic share count is 1

[Giuseppe Larosa · Accepted solution]

Hello Ashvanth,

OSPF costs are calculated in the outbound direction to the IP prefix, in your scenario you have different costs in different directions. This is generally not used in real world.

OSPF uses the Router LSAs of neighbors to calculate a cost to the ABR router that generated the LSA type 3 summary route.

So RA uses its  own Router LSA and cost of 3 to reach RB, then uses RB's router LSA to calculate a cost of 1 between RB and Rc (the ABR).

So, from the point of view of router A in your network diagram the cost to the ABR is 3+1 = 4 and I guess the cost of the summary route as advertised by router C in your network diagram is 1.

This ends up with a total cost of 5 to the inter area prefix from RA.

Hope to help

Giuseppe

[Joseph W. Doherty]

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Ditto.

[ashvanth kumar]

Thx Giuseppe Larosa !!!!!!!!!