C H A P T E R42
Identify the metrics that IGRP uses to compare routes.
• Explain how an administrator can influence route selection.
• Understand multipath routing.
Identify IGRP’s native stability features.
Identify IGRP’s timing mechanisms and explain their purpose.
Interior Gateway Routing Protocol
The Interior Gateway Routing Protocol (IGRP) is a routing protocol that was developed in the
mid-1980s by Cisco Systems, Inc. Cisco’s 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
In the mid-1980s, the most popular Interior Gateway Routing Protocol was the Routing Information
Protocol (RIP). 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) support of only
equal-cost load balancing (in all-Cisco networks only!) did not allow for much 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. IGRP was designed to
run in any network environment, however, and Cisco soon ported it to run in OSI
Connectionless-Network Protocol (CLNP) networks. Cisco developed Enhanced IGRP in the early
1990s to improve the operating efficiency of IGRP. This chapter discusses IGRP’s basic design and
implementation. Enhanced IGRP is discussed in Chapter 40, “Enhanced IGRP.”
IGRP Protocol Characteristics
IGRP is a distance vector Interior Gateway Protocol (IGP). Distance vector routing protocols
mathematically compare routes using some measurement of distance. This measurement is known as the
distance vector. Routers using a dis