Best Path and Metric

Determining a router's best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or "shortest" path to reach that network. Whenever multiple paths to reach the same network exist, each path uses a different exit interface on the router to reach that network. The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. Some routing protocols, such as RIP, use simple hop-count, which the number of routers between a router and the destination network. Other routing protocols, such as OSPF, determine the shortest path by examining the bandwidth of the links, and using the links with the fastest bandwidth from a router to the destination network.

A metric is the quantitative value used to measure the distance to a given route. The best path to a network is the path with the lowest metric. Two metrics that are used by some dynamic routing protocols are:

• Hop count-Hop count is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of four indicates that a packet must pass through four routers to reach its destination. If multiple paths are available to a destination, the routing protocol, such as RIP, picks the path with the least number of hops.

• Bandwidth-Bandwidth is the data capacity of a link, sometimes referred to as the speed of the link. For example, Cisco's implementation of the OSPF routing protocol uses bandwidth as its metric. The best path to a network is determined by the path with an accumulation of links that have the highest bandwidth values, or the fastest links. 

Equal Cost Load Balancing

You may be wondering what happens if a routing table has two or more paths with the same metric to the same destination network. When a router has multiple paths to a destination network and the value of that metric (hop count, bandwidth, etc.) is the same, this is known as an equal cost metric, and the router will perform equal cost load balancing. The routing table will contain the single destination network but will have multiple exit interfaces, one for each equal cost path. The router will forward packets using the multiple exit interfaces listed in the routing table.