FastEthernet, GigabitEthernet, and TenGigabitEthernet links will have the same OpenShortest Path First (OSPF) cost. An OSPF routing process uses a cost metric that is based on the bandwidth of an interface relative to a reference bandwidth. The formula to determine the cost of an interface is as follows: cost = reference bandwidth / interface bandwidth The default reference bandwidth is 100 Mbps. You can issue the autocost command from router configuration mode to change the reference bandwidth for an OSPF routing process. The syntax for the autocost command is autocost reference bandwidth refbw, where refbw is the reference bandwidth expressed as an integer value in megabits per second between 1 and 4294967. Therefore, the default value of the refbw parameter is 100.

The minimum supported cost for an OSPF interface is 1, and any values that calculate to less than 1 are rounded up to 1. Therefore, any link with an interface bandwidth greater than or equal to 100 Mbps will result in a cost of 1 by default. As a result, the 100Mbps FastEthernet links, the 1Gbps GigabitEthernet links, and the 10Gbps TenGigabitEthernetlinks in this scenario will all have a cost of 1; the 10Mbps Ethernet links will have a cost of 10.

If the reference bandwidth is less than the fastest routed link on the network, a situationcan arise where the cost of two interfaces is the same even though their link speeds are different. When an OSPF routing process is presented with multiple routes of the same cost, equalcost load balancing is used to distribute packets evenly among the available paths. This distribution will cause some packets in this scenario to take suboptimal routes to their destinations. To prevent this from occurring, the reference bandwidth should be a value greater than or equal to the bandwidth of the fastest routed link in the administrative domain. Alternatively, you can manually configure an OSPF cost for each interface by issuing the ip ospf cost command from interface configuration mode.