Content

| 3.5 |  | Route Calculation |
| 3.5.3 |  | Routing metrics |

A routing metric is a value that measures desirability. Some routing protocols use only one factor to calculate a metric. IP RIP uses hop count as the only factor to determine the metric of a route. Other protocols base their metric on two or more factors, including hop count, bandwidth, delay, load, reliability, and maximum transmission unit (MTU).

Bandwidth and delay are static. They remain the same for each interface until the router is reconfigured or the network is redesigned. Load and reliability are dynamic and calculated for each interface in real time by the router.

The more factors that make up a metric, the greater the ability is to adapt network operation to meet specific needs. IGRP, by default, uses two static factors to calculate the metric, bandwidth and delay. These can be configured manually, allowing 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 changing conditions. Therefore, if a link becomes heavily loaded or unreliable, IGRP will increase the metric of routes using that link and alternate routes presenting a lower metric would be used.

| Web Links Routing Basics http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/routing.htm