BW = 10 7/(min bandwidth along paths in kilobits per second)
Delay = (Sum of delays along paths in milliseconds)/10
Load = Load of interface
Reli = Reliability of the interface
From the equation, the load variable is a value from 1 to 255, in which 255 indicates 100 percent saturation of the link and 1 indicates virtually no traffic. The reli variable is also a value from 1 to 255, in which 1 indicates an unreliable link and 255 indicates a 100 percent reliable link.
Referring to Equation 4-1, the term K5 /(Reli + K4) is used only if K5 is not equal to 0. If K5 is equal to 0 (as the default), the term K5 /(Reli + K4) is ignored in the equation.
Variables K1 through K5 are constant numbers used in the equation. The default value of the K values are: K1 = K3 = 1, K2 = K4 = K5 = 0. The IGRP metric equation then reduces to this:
IGRP Metric = BW + Delay
Therefore, by default, IGRP considers only the bandwidth and the delay of the link to calculate its metrics. The network administrator can change the default K value to other numbers so that other components of the metric equation, such as load and reliability, can be used. For example, if the network administrator wants to consider interface reliability as one factor in routing the packet, the value of K5 would have to be a nonzero number; however, such a change is highly not recommended.
The bandwidth variable is the minimum bandwidth along the path from the local router to the destination, in kilobits per second, scaled by 107. The bandwidth associated with an interface is a static value assigned by the router or a network administrator; it is not a dynamic value that changes with throughput. The minimum bandwidth is obtained by comparing the interfaces along the paths to the destination network. For example, a network that needs to traverse a T1 link and an Ethernet link will have a minimum bandwidth of 1544 kbps. Notice that the bandwidth on a regular serial interface is assumed to be T1 with a speed of 1544 kbps.
The delay variable is the sum of all delays along the interfaces crossed in the path to the destination, in microseconds, divided by 10. Therefore, the delay variable used in IGRP metric equation has the unit of tens of microseconds. Like the bandwidth variable, the delay associated with each interface is a static value assigned by the router or a network administrator; it is not a dynamic value that changes with different traffic pattern. Table 4-1
lists router default bandwidth and delay values for some common interfaces.
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