Chapter 4. Understanding Interior Gateway Routing Protocol (IGRP)

This chapter covers the following key topics about Interior Gateway Routing Protocol (IGRP):

• Metrics

• Timers

• Split horizon

• Split horizon with poison reverse

• IGRP packet format

• IGRP behavior

• Default route and IGRP

• Unequal -cost load balancing in IGRP

In the mid-1980s, Cisco developed its own proprietary routing protocol, Interior Gateway Routing Protocol (IGRP), as a solution to some of the shortcomings of RIP, such as the hop-count limitation of 15.

Like RIP, IGRP is a distance vector protocol. However, IGRP calculates its composite metric from a variety of variables , such as bandwidth and delay, and hop count is not considered in the routing decision. IGRP uses variables such as interface bandwidth and delay, which reflect a better picture of the network topology. This results in a more efficient method of routing packets. Other advantages of IGRP over RIP include unequal-cost load sharing; a longer up-date period than RIP, for better bandwidth usage; and a more efficient packet-update format.