Metric Calculation

IGRP calculates and uses a composite metric of the previous values to calculate the route optimization. The metric value ranges from 1 to 16,277,215 (2²⁴ 1), and for purposes of route calculation, the lower the number (closer to zero), the better the route.

This composite metric reflects the various weights of each of the previously listed metrics. The general formula for this composite metric is as follows:

 Metric = (K1 x Bandwidth) + (K2 x Bandwidth)/(256   Load) + (K3 x Delay) 

K1, K2, and K3 are constants used to weigh the effect of these routing metrics; the default values for K1 and K3 is 1, and the default for K2 is 0.

There are two other constant values, K4 and K5, both of which default to 0 and are not used if they are left at the default value. The network administrator can change these values because they are not changed dynamically by IGRP operation.

The mathematical simplification of the composite metric, provided that all values remain at their defaults, is as follows:

 Metric = Bandwidth + Delay 

The reliability metric, K5, is a constant and is used only if the reliability metric is greater than the default of 0. The composite operation used to determine the metric for K5 (Reliability) > 0 is shown here:

 Metric = Metric x [K5/(Reliability + K4)] 

Delay is in units of 10 microseconds. This gives a range of 10 microseconds to 168 seconds.

Bandwidth is the inverse minimum bandwidth of the path in bits per second scaled by a factor of 10e10. The range is from a 1200bps line to 10Gbps.

Table E-1 lists the default delay values used by IGRP.

Table E-1. IGRP Default Delay Values

Media	Delay	Bandwidth
Satellite	200,000 (2 sec)	20 (500 Mbit)
Ethernet	100 (1 ms)	1,000
1.544 Mbit	2000 (20 ms)	6,476
64 Kbit	2000 (20 ms)	156,250
56 Kbit	2000 (20 ms)	178,571
10 Kbit	2000 (20 ms)	1,000,000
1 Kbit	2000 (20 ms)	10,000,000