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IGRP (Interior Gateway Routing Protocol)

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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.

IGRP Operation

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:

  • Upon router startup, IGRP broadcast request messages to other routers. Upon receipt of the message, router send their routing tables to the startup router. Routing tables contain entries for each of the networks that a router can reach.
  • At regular intervals, routers automatically transmit their routing tables to other routers which in turn update their own routing tables if there are changes in the topology.
  • A triggered update occurs if the network topology changes. Triggered updates allow quick responses to changes in the network.
  • When routers add entries to routing tables, 1 is added to the hop count to account for the hop between the router and the neighbor from which it received the route information.
  • When a route is entered into the routing table, a timer is set. As routing table updates arrive and a route entry is verified to still be valid, the timer is reset. If a known router fails to appear in an update and in subsequent updates, the timer will run out and the route entry will be purged.

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.

  • Internetwork delay This metric is based on the delay of different types of networks and links (token ring, Ethernet, T1, etc.), which Cisco has predefined.
  • Bandwidth This metric is based on the bandwidth of different types of networks and links (token ring, Ethernet, T1, etc.), which Cisco has predefined.
  • Reliablity A metric that uses a scale of 1 to 255, where 255 is a 100% reliable interface.
  • Load A metric that uses a scale of 1 to 255, where 255 is a 100% reliable load.

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:

  • Backward compatible with IGRP, allowing EIGRP to be introduced into existing IGRP networks.
  • EIGRP provides fast convergence since routers store all neighbor routing tables to promote faster route updates if necessary
  • Supports variable-length subnet mask, allowing route summarization (aggregation) and the creation of network boundaries
  • Supports partial updates so that entire routing tables do not need to be sent at regular intervals. Updates are sent when a route changes and the changes are sent to only the routers that need the information.
  • Supports multiple network-layer protocols, including AppleTalk, Novell NetWare, and IP. The IP implementation of EIGRP redistributes reoutes learned from OSPF, RIP, IS-IS, EGP, and BGP.

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.
All rights reserved under Pan American and International copyright conventions.