Split Horizon

Chapter 10. Distance Vector Protocols: Interior Gateway Routing Protocol (IGRP)

When Cisco Systems developed the Interior Gateway Routing Protocol (IGRP) around 1986, network administrators didn't have many options to deal with some of RIP's limitations. RIP's hop-count limit of 15 and its simplistic metrics weren't allowing networks to scale and distribute traffic across paths of unequal cost. OSPF would not come out for another two years , and another routing protocol was needed. As the pioneer of internetworking, Cisco developed IGRP to specifically address some of RIP's shortcomings.

IGRP has features that differentiate it from other distance vectors protocols:

• Scalability ” A hop count limit of 255 provides a broader network diameter versus RIP's hop-count limit of 15. The default hop for IGRP is 100.

• Faster convergence ” IGRP uses Flash updates, which are updates that are sent to neighboring routers when topology changes occur.

• Sophisticated metric ” IGRP uses a composite metric based on five individual metrics ”bandwidth, delay, reliability, load, and MTU ”to influence routing decisions.

• Unequal-cost load balancing ” IGRP composite routing metrics allow for load balancing across multiple unequal-cost paths.

These features provided significant enhancements over the routing protocols of the mid-1980s. But eventually, like RIP, IGRP networks will give way to their stronger brethren, Enhanced IGRP. This chapter covers the features and operation of IGRP, along with configuration and redistribution of IGRP.