Editing Interior Gateway Routing Protocol

{{IPstack}}
'''Interior Gateway Routing Protocol''' ('''IGRP''') is a [[distance vector]] [[interior gateway protocol]] (IGP) developed by [[Cisco]]. It is used by [[Router (computing)|router]]s to exchange [[routing]] data within an [[autonomous system (Internet)|autonomous system]].

IGRP is a [[proprietary protocol]].  IGRP was created in part to overcome the limitations of [[Routing Information Protocol|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 (computing)|bandwidth]], [[Network delay|delay]], [[Load (computing)|load]], and [[Reliability (computer networking)|reliability]]; to compare two routes these metrics are combined into a single metric, using a formula which can be adjusted through the use of pre-set constants. By default, the IGRP composite metric is a sum of the segment delays and the lowest segment bandwidth. The maximum configurable hop count of IGRP-routed packets is 255 (default 100), and routing updates are [[Broadcasting (networking)|broadcast]] every 90 seconds (by default).<ref>[http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfigrp.html Cisco Systems] Configuring IGRP</ref> IGRP uses [[List of IP protocol numbers|protocol number]] 9 for communication.<ref>{{cite web
|url=https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xml
|title=Assigned Internet Protocol Numbers
|publisher=[[Internet Assigned Numbers Authority]] (IANA)
|access-date=18 June 2013}}</ref>

IGRP is considered a [[classful]] routing protocol. Because the protocol has no field for a [[subnet mask]], the router assumes that all subnetwork addresses within the same Class A, Class B, or Class C network have the same subnet mask as the subnet mask configured for the interfaces in question. This contrasts with classless routing protocols that can use [[Classless Inter-Domain Routing|variable length subnet masks]]. Classful protocols have become less popular as they are wasteful of [[IPv4 address exhaustion|IP address space]].

==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. In the new Cisco [[CCNA]] curriculum (version 4), IGRP is mentioned only briefly, as an "obsolete protocol".

==References==
<references/>

==External links==
* [http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a00800c8ae1.shtml Cisco - An Introduction to IGRP]
* [https://patents.google.com/patent/US5088032 US Patent 5,088,032 "Method and apparatus for routing communications among computer networks"]

[[Category:Cisco protocols]]
[[Category:Routing protocols]]