Summary

IGRP is a proprietary routing protocol that was developed by Cisco Systems, Inc. in the early 1990s and was built with functional similarity to RIP, but with the additional features of weighted metrics.

To enable networks to achieve greater efficiency, size, stability, and control over resources, IGRP uses a composite metric calculated by factoring weighted mathematical values for network delay, bandwidth, reliability, and load.

These metric ranges are complemented by user-definable constants, as noted previously and in the list that follows (k1 k5), enabling a network administrator to influence route selection. These constants are used in an algorithm yielding a single, composite metric. This composite metric enables the network administrator to influence route selection by giving higher or lower weighting to specific metrics. This flexibility enables administrators to fine-tune IGRP's automatic route selection.

IGRP permits multipath routing, enabling dual equal-bandwidth lines to run a single stream of traffic in round-robin fashion, with automatic switchover to the second line if one line goes down. Multiple paths can have unequal metrics and still be valid multipath routes. For example, if one path is three times better than another path (its metric is three times lower), the better path will be used three times as often.

Cisco Systems developed an enhancement to IGRP, aptly named the Enhanced Interior Gateway Routing Protocol (EIGRP) in the early 1990s.

The composite IGRP metric is computed according to the following formula:

Metric = (K1 x Bandwidth) + (K2 x Bandwidth)/(256 Load) + (K3 x Delay) x [K5/( graphics/ccc.gifReliability + K4)]

The default K values are as follows:

  • K1 = 1

  • K2 = 0

  • K3 = 1

  • K4 = 0

  • K5 = 0

  • If K5 = 0, there is no reliability term.

IGRP has proven to be one of the most successful routing protocols. No small part of its success has been due to its functional similarity to RIP, a simple yet highly successful and widely deployed routing protocol. Cisco took great pains to carefully preserve many of the effective features of RIP, while greatly expanding its capabilities. Today, IGRP is showing its age; it lacks support for variable-length subnet masks (VLSM). Rather than develop an IGRP version 2 to incorporate that capability, Cisco has built upon IGRP's legacy of success with Enhanced IGRP.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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