Appendix E. Interior Gateway Routing Protocol (IGRP)

The following topics are covered in this appendix:

• IGRP Metrics

• Metric Calculation

• IGRP Timers

• IGRP Convergence Mechanisms

• IGRP Load Balancing

Cisco Systems developed the Interior Gateway Routing Protocol (IGRP) in the early 1980s as an answer to the limitations of RIPv1 (Routing Information Protocol, version 1), while preserving its ease of implementation.

The principal goal in creating IGRP was to provide a robust protocol for routing within an autonomous system (AS). Such protocols are known as Interior Gateway Routing Protocols. Although RIP was quite useful for routing within small- to moderate-sized, relatively homogeneous internetworks, its limits were being pushed by network growth. In particular, RIP's small hop-count limit (16) restricted the size of internetworks; single metric (hop count) did not allow for routing flexibility in complex environments.

The popularity of Cisco routers and the robustness of IGRP encouraged many organizations with large internetworks to replace RIP with IGRP. Cisco's initial IGRP implementation worked in Internet Protocol (IP) networks.