Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 123
Metrics (Cost)

A cost is associated with the output side of each router interface. This cost is a configurable parameter on the router. When LSAs are transferred between routers, the cost of the individual links is added as well. The cost of a link is the cost associated on the outbound link and this information is added up in a router (receiving LSAs) before Dykstra runs. Multiple paths can be found to a destination and the path with the lowest cost will be placed in the routing table. Simply stated, the lower the cost of a router port, the more likely the interface is to be used to forward data traffic.

According to RFC 1253 (OSPF Version 2 MIB), the following is a recommendation for assigning costs to links in an OSPF environment:

For costing a link, there is a default value that can be used. It is only a recommendation and any number can be used. For example, if you are using a higher-speed link (such as those available with the ATM protocol) room should be left to compensate for this. This yields a number having the following typical values:

Metric = 10n8 /interface speed

Network type/bit rate—Metric