Site home page
Get alerts when Linktionary is updated
Book updates and addendums
Get info about the Encyclopedia of Networking and Telecommunicatons, 3rd edition (2001)
Download the electronic version of the Encyclopedia of Networking, 2nd edition (1996). It's free!
Contribute to this site
Electronic licensing info
IGRP (Interior Gateway Routing Protocol)
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
IGRP is a Cisco interior routing protocol based on distance-vector routing. An interior routing protocol is meant to be used inside an autonomous system (an organization's private network) while an exterior routing protocol operated between autonomous systems. IGRP is a distance-vector protocol, as opposed to a link-state protocol. While link-state protocols are superior, distance-vector protocols are appropriate for small internetworks, and require much less configuration and management. See "Distance-Vector Routing" for more information.
Cisco developed IGRP in the 1980s to provide an alternative to RIP (Routing Information Protocol). At the time, IGRP was a significant improvement over RIP, which had a hop count restriction that limited the size of an internetwork. IGRP supports internetworks with up to 255 hops.
Alternatives to IGRP are EIGRP (Enhanced Interior Gateway Routing Protocol) and OSPF (Open Shortest Path First). EIGRP is a distance-vector routing protocol similar to IGRP, but with many enhancements. OSPF is an IETF-developed link-state routing protocol that is suitable for large internetworks and the Internet. OSPF is now the preferred interior routing protocol.
Being a distance vector routing protocol, IGRP shares many features in common with RIP. This section will focus on the differences. For details about basic operation, refer to "Distance Vector Routing" and "RIP (Routing Information Protocol)." The basic operation is outlined below:
Distance vector routing is based on distance. A distance vector table is built by each router that contains two primary entries: a vector (destination) and a distance (cost). IGRP adds more flexibility to this model by supporting several different types of cost metrics as outlined below. Administrators set metrics based on their own requirements, or else IGRP uses default metrics. Routers use these metrics to calculate the best route to a destination.
IGRP also supports multipath routing of up to six parallel paths.
For additional information, be sure to read "An Introduction to IGRP," by Charles L. Hedrick, available at the Cisco Web site listed on the related entries page.
EIGRP (Enhanced IGRP)
According to the paper "Enhanced IGRP" at Cisco's Web site (address listed later) (Enhanced IGRP integrates the capabilities of link-state protocols into distance-vector protocols. It incorporates the Diffusing-Update Algorithm (DUAL) developed at SRI International by Dr. J.J. Garcia-Luna-Aceves." The protocol has these additional features:
EIGRP uses neighbor discovery/recovery to dynamically learn about other routers on directly attached networks. Hello packets are periodically sent to monitor the status of links. EIGRP also uses a reliable transport protocol to provide guaranteed, ordered delivery of packets to neighbors. Route computations are handled by DUAL, which uses distance information to select efficient, loop-free paths.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.