Metric and administrative distance

The routing table contains the single best route to each destination that the router has learned. However, a router may learn more than one route to the same destination. The router compares the metrics and administrative distances of these routes to select the best route to add to its routing table.

• Administrative distanceThe routing switch uses this parameter to compare routes learned by different routing methods. It indicates how reliable the router considers the method through which it discovered the route: a lower value indicates a more trustworthy route.Administrative distance is not a factor if you are using only static routes. However, if you are using static routing in conjunction with a routing protocol such as OSPFv3 to provide routes to an identical destination, the routing switch selects the route with the lowest administrative distance. Where the default administrative distances are used, a static route normally supersedes a dynamic route to the same destination because the former has the lowest default administrative distance and metric.

  Routing method	Administrative distance	Metric
  Direct connection	0 (not configurable)	1
  Static route	Default: 1; range: 1 to 255	1
  OSPFv3	Default: 110 (external, inter-area, and intra-area) range (for all three): 1 to 255	Variable

• Metric: The routing switch uses this parameter to compare routes to identical destinations learned by the same routing protocol. The metric is the cost of sending traffic on a given route and is based on various criteria:

  • Link conditions (bandwidth, delay, reliability)

  • Organizational policies (monetary cost, autonomous systems that a packet must traverse)Each routing protocol has its own method for computing a route's metric. For static routes, the metric defaults to "1" and is not configurable.