Introduction to Interior Gateway Routing Protocol (IGRP)

The Interior Gateway Routing Protocol (IGRP) is a Cisco-proprietary routing protocol for IP.

Some features of Interior Gateway Routing Protocol (IGRP) are

• Interior Gateway Routing Protocol (IGRP) uses a sophisticated metric based on bandwidth and delay.

• Interior Gateway Routing Protocol (IGRP) uses triggered updates to speed-up convergence.

• Interior Gateway Routing Protocol (IGRP) supports unequal-cost load balancing to a single destination.

Interior Gateway Routing Protocol (IGRP) uses bandwidth, delay, reliability, load, to find the metric value. By default, the algorithm uses only bandwidth and delay, but the other metric components can be enabled. IGRP uses bandwidth, delay, reliability, load, and MTU to find the metric value. By default, the algorithm uses only bandwidth and delay, but the other metric components can be enabled.

The following formula is used to calculate the composite metric of IGRP.

Metric = [K1 * Bandwidth + (K2 * Bandwidth)/ (256-Load) + K3*Delay] * [K5/(Reliability + K4)]

The default constant values are K1 = K3 = 1 and K2 = K4 = K5 = 0.

If K5 = 0, the [K5/ (reliability + K4)] term is not used. So, given the default values for K1 through K5, the composite metric calculation used by IGRP reduces to Metric = Bandwidth + Delay.

• To find the bandwidth value, find the smallest of all the bandwidths in Kbps from outgoing interfaces and divide 10,000,000 by that number.

• Reliability and load are measured 1–255. A reliability of 1 is least reliable, while 255 is most reliable. A load of 1 is least utilized, while 255 is 100 percent utilized. The MTU refers to the size of the frame. If a route has lower metric value, then that route is preferred.

• In order to find the delay, add all of the delays (in microseconds) from the outgoing interfaces and divide this number by 10. (The delay is in tenths of microseconds.)