Cisco Systems CCNA 2 manual Igrp features, Igrp metrics

7.3.1 IGRP features

IGRP is a Cisco proprietary distance-vector interior gateway routing protocol. Distance vector routing protocols mathematically compare routes to determine the best path. IGRP was designed to take advantage of the simplicity of RIP and adds other metrics for best path selection and better scalability. The metrics available with IGRP are bandwidth, delay, load, reliability, and maximum transmission unit (MTU). These metrics can be used to make better mathematical decisions about best paths than the hop count metric used by RIP. By default, bandwidth and delay are the two metrics that are used and the others are set to zero. IGRP shares its routing information through the use of timed updates every 90 seconds.

Draw an example on the board to demonstrate how IGRP can make better routing decisions than RIP. There are three important points to emphasize:

•IGRP is Cisco proprietary. If students can select which routing protocols to use, the internetwork will have to be all Cisco devices for IGRP to be chosen.

•The default update time of IGRP is 90 seconds and the updates are broadcast.

•The default algorithms of IGRP are bandwidth and delay. The others can be used if the algorithm is changed. MTU is only exchanged in the update. It is not used in any calculation.

One additional item to note is that Cisco offers more support for EIGRP than IGRP. Many of the newer releases of IOS do not support IGRP.

7.3.2 IGRP metrics

IGRP uses several metrics to calculate the overall routing metric of each route:

•Bandwidth – the lowest bandwidth value in a path

•Delay – the cumulative interface delay along a path

•Reliability – the reliability between source and destination, determined by the exchange of keepalives

•Load – the load on a link between a source and a destination based on bits per second

•MTU – the maximum transmission unit value of a path

The show ip protocol command is used to display parameters, filters, and network information about the routing protocol used by a router. Each metric has a corresponding K value or weight. By default, only K1 and K3 are set to one. These represent the K values for bandwidth and delay. The K values of the other metrics are set to zero. By default, only bandwidth and delay are used to determine the composite metric or routing metric of each route. This use of multiple components to calculate a composite metric provides greater accuracy than the RIP hop-count metric to choose the best path.

The show ip route command displays the composite IGRP metric for a given route in brackets with the administrative distance. A link with higher bandwidth will have a lower metric. A link with lower cumulative delay will have a lower metric.

87 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7 Copyright ♥ 2004, Cisco Systems, Inc.