Internet Gateway Routing Protocol (IGRP)

IGRP is a Cisco proprietary routing protocol that was designed as a new version or upgrade from RIP, the only existing protocol at the time. IGRP does not use RIP's hop count of 15; the IGRP routing protocol uses up to 255 hops. IGRP is a classful routing protocol, meaning it does not include any subnet information about the network.

Three types of routes are recognized by IGRP:

• Interior ” A network directly connected to a router interface.

• System ” Advertised routes by other IGRP neighbors within the same AS (autonomous system) number. The AS is the number that identifies the IGRP individual session.

• Exterior ” Routes learned from other IGRP sessions with different AS numbers . This information is used by the router to set the gateway of last resort ”the route a packet takes if it is not specified to a certain route.

IGRP also uses more than just hop counts to determine the best path from or to a network. It uses a combination of internetwork delay, bandwidth, link reliability, and load to determine the best path on the network. IGRP also uses split horizon, poison reverse, hold-down timers, and triggered updates, as described earlier in this chapter. When using IGRP, if routing metrics have increased by 1.1 or more, the poison -reverse update is started. The reason for this is if there is an increase in the routing metrics, than there might be a routing loop. Not unlike RIP, the IGRP routing protocol also implements a number of timers.

Troubleshooting IGRP

IGRP uses distance-vector routing and, because of this, uses a one-dimensional array of information to calculate the best path. The vector consists of four elements: bandwidth, delay, load, and reliability. The MTU, or maximum transfer unit, is not part of the vector of metrics but is used in the final route information. IGRP is intended to replace RIP in order to create a more stable, quick-converging protocol that can scale up to a growing network.

If you implement a large-scale network, you may want to use the link-state protocol, because the distance-vector protocols come with overhead and delay time.

The IGRP protocol has a few items that provide a quick convergence time:

• Configurable metrics ” Metrics involved in the algorithm responsible for calculating route information may be configured by the user .

• Flash updates ” Updates are sent out immediately when the metrics route changes, there is no waiting for routine updates.

• Poison reverse ” Used to prevent routing loops .

• Unequal-cost load balancing ” Allows packets to be changed/distributed across multiple paths.