# Interior Gateway Routing Protocol

The Interior Gateway Routing Protocol ( IGRP ) is a proprietary distance-vector routing protocol developed by Cisco in the 1980s that is used by routers to exchange routing information within an autonomous system .

The main goals in the development of IGRP were to improve scalability and to overcome the maximum number of 15 network nodes specified by RIP that a target network may be away from before the network is considered to be inaccessible. The maximum number of routers between the starting point and destination ("hops") with IGRP is 255.

IGRP can use the available bandwidth, the delay occurring on the path, the line reliability and the line utilization to generate cost metrics. At IGRP, the metric is calculated using the formula

${\ displaystyle {\ text {Metric}} = {\ begin {cases} \ left (K_ {1} \ BW _ {\ text {IGRP}} + {\ frac {K_ {2} \ BW _ {\ text {IGRP} }} {256-LOAD _ {\ text {IGRP}}}} + K_ {3} \ DLY _ {\ text {IGRP}} \ right) \ left ({\ frac {K_ {5}} {RLY _ {\ text { IGRP}} + K_ {4}}} \ right) & {\ text {if}} K_ {5}> 0 \\ K_ {1} \ BW _ {\ text {IGRP}} + {\ frac {K_ {2 } \ BW _ {\ text {IGRP}}} {256-LOAD _ {\ text {IGRP}}}} + K_ {3} \ DLY _ {\ text {IGRP}} & {\ text {if}} K_ {5} = 0 \ end {cases}}}$

${\ displaystyle BW _ {\ text {IGRP}} = {\ frac {10 ^ {7}} {bandwidth [kbps]}}}$, for example on a T1 line at 1544kbps ${\ displaystyle BW _ {\ text {IGRP}} = {\ frac {10 ^ {7}} {1544}} = 6476}$

${\ displaystyle DLY _ {\ text {IGRP}}}$ is the delay time in 10 microsecond units, for example 20000 microsecond delay ${\ displaystyle DLY _ {\ text {IGRP}} = {\ frac {20000} {10}} = 2000}$

By default, the cost metric of a route is formed from the bandwidth and the line delay, i. H. and and . ${\ displaystyle K_ {1} = K_ {3} = 1}$${\ displaystyle K_ {2} = K_ {4} = K_ {5} = 0}$${\ displaystyle {\ text {Metric}} = BW _ {\ text {IGRP}} + DLY _ {\ text {IGRP}}}$

Split horizon processes, holddown timers , route poisoning and triggered updates are used to avoid routing loops .

IGRP will not be further developed and is no longer supported as of Cisco IOS v12.3. The EIGRP protocol, which in contrast to IGRP supports VLSM , for example , was used by Cisco as its successor.