OSPF uses cost as the metric for
determining the best route. Cost is calculated using the formula 108/bandwidth,
where bandwidth is expressed in bps. The Cisco
IOS automatically determines cost based on the bandwidth of the
interface.  It is essential for proper OSPF operation that the correct
interface bandwidth is set.
Router(config)#interface
serial 0/0
Router(config-if)#bandwidth 64
The default bandwidth for Cisco serial
interfaces is 1.544 Mbps, or 1544 kbps.
Cost can be changed to influence the
outcome of the OSPF cost calculation. A common situation requiring a
cost change is in a multi-vendor routing environment. A cost change
would ensure that one vendor’s cost value would match another vendor’s
cost value. Another situation is when Gigabit Ethernet is being used.
The default cost assigns the lowest cost value of 1 to a 100 Mbps link.
In a 100-Mbps and Gigabit Ethernet situation, the default cost values
could cause routing to take a less desirable path unless they are
adjusted. The cost number can be between 1 and 65,535.
Use the following interface configuration
command to set the link cost:
Router(config-if)#ip
ospf cost
number
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Lab Activity
Lab Exercise: Modifying OSPF Cost Metric
This lab is to setup an Open Shortest Path First (OSPF) area.
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Lab Activity
e-Lab Activity: Modifying OSPF Cost Metric
In this lab, the student will modify the OSPF cost metric.
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