1.10.d OSPF Path preference

The cost of an OSPF link is calculated by dividing the reference bandwith by the interface bandwith.

• The default reference bandwith is 100,000,000 bits per second, or 100 Mbps. This can be changed.

  • If you were to calculate the cost of a 100Mpbs link, your cost for that link would be 1.

• 1 Mbps - 100

• 10 Mbps - 10

• 100 Mbps - 1

• 1 Gbps - By default anything over 100mbps is also 1.

• The most efficient path from PC1 to PC2 in this case is from R1 > R2 > R3. This would have a cost of 3.
  • Taking the path from R1 straight to R3 would cost 11.

Adjusting reference bandwidth

The following command changes the default reference bandwidth used in OSPF calculations.

• Router(config-router)# auto-cost reference-bandwidth <bandwidth in Mbps>

The following command changes the default reference bandwidth used in OSPF calculations to 1 gbps.

• Router(config-router)# auto-cost reference-bandwidth 1000