OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

5.3. Metric Types

Both OSPF and IS-IS use a dimensionless metric that is best named cost. The metric is assigned to every interface on which the protocol runs, either administratively or by some default value, and is essential for determining the shortest path between any two nodes. When evaluating the route from some source A to some destination Z, the cost of the route is the sum of the costs of all outgoing router interfaces from A to Z.

5.3.1. OSPF Metrics

RFC 2328 and its predecessors do not specify or suggest a default interface cost for OSPF, other than that it must be greater than 0. That leaves the potential for implementations to vary widely in their default value. Fortunately, many vendors have copied Cisco Systems’ method for determining ...