Metrics

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IGRP uses an equation to calculate its metrics. Metrics then are used by the router to favor a particular route. In IGRP, the lower the value of the metric is, the more favorable the route is. The IGRP metric equation takes into consideration the variables of bandwidth, delay, load, and reliability of the link to calculate its metric. Equation 4-1 shows the IGRP metric equation.

Equation 4-1 IGRP Metric Equation


K1, K2, K3, K4, K5 = Constants

Default values: K1 = K3 = 1, K2 = K4 = K5 = 0

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:


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 10 7 . 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.

Table 4-1. Router Default Bandwidth and Delay Values for Common Interfaces
Media Bandwidth Delay
Ethernet 10,000 kbps 1000 microseconds
Fast Ethernet 100,000 kbps 100 microseconds
Gigabit Ethernet 1,000,000 kbps 10 microseconds
FDDI 100,000 kbps 100 microseconds
Token Ring (16 M) 16,000 kbps 630 microseconds
T1 1544 kbps 20,000 microseconds

IGRP metric calculation is illustrated in the following example. Consider the network in Figure 4-1.

Figure 4-1. IGRP Metric Calculation Example

Now calculate the IGRP metric to Network X from Router 3's perspective. The lowest bandwidth to Network X is the T1 link with a total delay of 21,100 microseconds (100 micro-seconds + 20,000 microseconds + 1000 microseconds). Assume that all the K constant values are default values. Therefore, only the bandwidth and delay values are considered in calculating the IGRP metric. BW = 10 7 /1544 = 6476 and Delay = 21,100/10 = 2110. This yields IGRP metric = BW + Delay = 6476 + 2110 = 8586. Therefore, from Router 3's perspective, the IGRP metric to Network X is 8586.

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Troubleshooting IP Routing Protocols
Troubleshooting IP Routing Protocols (CCIE Professional Development Series)
ISBN: 1587050196
EAN: 2147483647
Year: 2002
Pages: 260

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