The Interior Gateway Routing Protocol (IGRP) is a routing protocol to provide routing within an autonomous system (AS). In the mid-1980s, the most popular interior 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. The popularity of Cisco routers and the robustness of IGRP encouraged many organizations with large internetworks to replace RIP with IGRP. Cisco developed Enhanced IGRP in the early 1990s to improve the operating efficiency of IGRP.

IGRP is a distance vector Interior Gateway Protocol (IGP). Distance vector routing protocols mathematically compare routes using some measurement of distance. Distance vector routing protocols are often contrasted with link-state routing protocols, which send local connection information to all nodes in the internetwork.

To provide additional flexibility, IGRP permits multipath routing. Dual equal-bandwidth lines can run a single stream of traffic in round-robin fashion, with automatic switchover to the second line if one line goes down. Multiple paths can have unequal metrics yet still be valid multipath routes.

Using IGRP, for example, if one path is three times better than another path (its metric is three times lower), the better path will be used three times as often. Only routes with metrics that are within a certain range or variance of the best route are used as multiple paths. Variance is another value that can be established by the network administrator.

Protocol Structure

• Version -- IGRP version number (currently 1).
• Opcode -- Operation code indicating the message type: 1 Update; 2 Request.
• Edition -- Serial number which is incremented whenever there is a change in the routing table.
• Asystem -- Autonomous system number. A gateway can participate in more than one autonomous system where each system runs its own IGRP. For each autonomous system, there are completely separate routing tables. This field allows the gateway to select which set of routing tables to use.
• Ninterior, Nsystem, Nexterior -- Indicate the number of entries in each of these three sections of update messages. The first entries (Ninterior) are taken to be interior, the next entries (Nsystem) as being system, and the final entries (Nexterior) as exterior.
• Checksum -- IP checksum which is computed using the same checksum algorithm as a UDP checksum.

Related Terms: IP, TCP, EIGRP, RIP, EGP, BGP

Sponsor Source: IGRP is a Cisco protocol.

Reference Links:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/igrp.htm: Cisco Interior Gateway Routing Protocol.