IGRP process and updates

IGRP (Interior Gateway Routing Protocol) is a Cisco proprietary protocol that works by providing neighboring routers with updates from internal routing tables. A number of factors, including hop count, hold down timers, reliability, load and delay, all make up factors in these routing tables.

The basic function of routing is the ability of a router to provide path determination – the function of evaluating the best possible path for routing a packet. The ways of calculating the “best path” vary between routing protocols; each protocol’s specific way of calculating the best path is called the routing “metric”. IGRP’s metrics are hop count, bandwidth, delay, load, reliability, cost and ticks. Out of these, the main determination factors IGRP uses are hop count, bandwidth, delay, reliability and load.

Hop count is the number of routers a packet has to go through to reach its destination, meaning the lower the hop count, the better choice the path is. Bandwidth is the actual capacity, or amount of traffic the line itself can carry. A line with a speed of 100Mbps is much better than a 56kbps line. Delay is the length of time that would be required to move the packet from one end of the network to the other, taking into consideration things like network traffic and priorities of the network. Reliability is the error rate of each network. Load is the amount of utilization of a link.

Good routing protocols such as IGRP also incorporate methods to improve the stability of the routing protocol and the network. These methods include holddowns, split horizons and poison reverse updates.

IGRP is a distance-vector protocol, which means it shares its routing table with neighboring routers to create knowledge of the topology of the network. When IGRP is used, an autonomous system (AS) network number must be assigned. The router then broadcasts its connected networks to other routers in the same AS.

Routing updates are sent every 90 seconds, this helps keep the routers up to date to network changes and allows for the best use of bandwidth. Usually the default settings for the other metrics of IGRP are set for the best possible bandwidth use. In our network, we will be setting maximum hop count to 10 (until requirements of the WAN set up changes the maximum hops needed), since we only have one router in our LAN. We are going to leave the default table updates at 90 seconds, both to keep the network traffic down and because we do not anticipate many network changes. We will also add “no metric holddown” so any potential new routes can be added instantly.

Serial interface bandwidth to be set at 56000 (clockrate). The AS number for our network is 150.

The network that will be advertised by our router is 20.0.0.0.

In order to configure the router for IGRP, here are the appropriate commands:

Router# config t
Router(config)# router igrp 150
Router(config-router)# network 20.0.0.0