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9.1 | |
Examining the Routing Table | |
| 9.1.6 | |
Determining the route metric |
Routing protocols use metrics to determine
the best route to a destination. The metric is a value that
measures the desirability of a route. Some routing protocols use only
one factor to calculate a metric. For example, RIP version 1 (RIP v1) uses hop count
as the only factor to determine the metric of a route. Other protocols
base their metric on hop count, bandwidth, delay, load, reliability,
ticks delay, maximum transmission unit (MTU), and cost.
 Each routing algorithm interprets what is best in its own way. The algorithm generates a number, called the metric value, for each path through the network. Typically, the smaller the metric number, the better the path. Factors such as bandwidth and delay are
static because they remain the same for each interface until the
router is reconfigured or the network is redesigned. Factors such as
load and reliability are dynamic because they are calculated for
each interface in real-time by the router.
The more factors that make up a metric, the greater the flexibility to tailor network operations to meet specific needs. By default, IGRP uses the static factors bandwidth and delay to calculate a metric value. These two factors can be configured manually, allowing precise control over what routes a router chooses. IGRP may also be configured to include the dynamic factors, load and reliability, in the metric calculation. By using dynamic factors, IGRP routers can make decisions based on current conditions. 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. IGRP calculates the metric by adding the weighted values of different characteristics of the link to the network in question. In the following example the values bandwidth, bandwidth divided by load, and delay, are weighted with the constants K1, K2, and K3. Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) + K3 * Delay The default constant values are K1=K3=1 and K2=K4=K5=0 so:
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