Interior Gateway Routing Protocol
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Interior Gateway Routing Protocol (IGRP) is a kind of IGP which is a distance-vector routing protocol invented by Cisco, used by routers to exchange routing data within an autonomous system.
IGRP was created in part to overcome the limitations of RIP (maximum hop count of only 15, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth, load, delay, MTU, and reliability; to compare two routes these metrics are combined together into a single metric, using a formula which can be adjusted through the use of pre-set constants. The maximum hop count of IGRP-routed packets is 255 (default 100).
IGRP is considered a classful routing protocol. As the protocol has no field for a subnet mask the router assumes that all interface addresses have the same subnet mask as the router itself. This contrasts with classless routing protocols that can use variable length subnet masks. Classful protocols have become less popular as they are wasteful of IP address space.
The protocol is unique in its treatment of default routes, or gateways of last resort. Rather than configuring a specific default gateway route, a network administrator must flag preexisting static routes as candidates for a default route. If two or more default route candidates exists, IGRP calculates the optimal default route based each route's metrics.
[edit] Advancement
In order to address the issues of address space and other factors, Cisco created EIGRP (Enhanced Interior Gateway Routing Protocol). EIGRP adds support for VLSM (variable length subnet mask) and adds the Diffusing Update Algorithm (DUAL) in order to improve routing and provide a loopless environment. EIGRP has completely replaced IGRP, making IGRP an obsolete routing protocol. In Cisco IOS versions 12.3 and greater, IGRP is completely unsupported. IGRP is still taught in Cisco's CCNA curriculum, but it should be noted that knowledge of IGRP is not tested.
