3.5 Route Calculation
3.5.3 Routing metrics
A routing metric is a value that measures the desirability. Some routing protocols use only one factor to calculate a metric. For example, IP RIP uses hop count as the only factor to determine the metric of a route. Other protocols base their metric on two, three, or even five different factors, such as hop count, bandwidth, delay, load, and reliability.

Some factors, such as bandwidth and delay, are static which means that they remain the same for each interface until the router is reconfigured or the network is redesigned. Other factors, such as load and reliability, are dynamic which means that they are calculated for each interface in real time by the router.

The more factors that make up a metric, the greater your ability is to tailor network operation to meet specific needs. For example, IGRP by default uses two static factors to calculate metric: bandwidth and delay. You can configure these two factors manually, which allows for precise control over what routes a router chooses. IGRP can also be configured to include two dynamic factors in the metric calculation: load and reliability. By using dynamic factors, IGRP routers can make decisions based on conditions at that moment. Thus, if a link becomes heavily loaded or unreliable, IGRP will increase the metric of routes using that link. Alternate routes may present a lower metric than the downgraded route and would be used instead.