- Posted at 15:02 on 20/1/2007 by @lfons
As mentioned above, IGRP is a protocol that allows gateways to build up their routing table by exchanging information with other gateways. A gateway starts out with entries for all of the networks that are directly connected to it. It gets information about other networks by exchanging routing updates with adjacent gateways. In the simplest case, the gateway will find one path that represents the best way to get to each network. A path is characterized by the next gateway to which packets should be sent, the network interface that should be used, and metric information. Metric information is a set of numbers that characterize how good the path is. This allows the gateway to compare paths that it has heard from various gateways and decide which one to use. There are often cases where it makes sense to split traffic between two or more paths. IGRP will do this whenever two or more paths are equally good. The user can also configure it to split traffic when paths are almost equally good. In this case more traffic will be sent along the path with the better metric. The intent is that traffic can be split between a 9600 bps line and a 19200 BPS line, and the 19200 line will get roughly twice as much traffic as the 9600 BPS line.
The metrics used by IGRP include the following:
· Topological delay time
· Bandwidth of the narrowest bandwidth segment of the path
· Channel occupancy of the path
· Reliability of the path
Topological delay time is the amount of time it would take to get to the destination along that path, assuming an unloaded network. Of course there is additional delay when the network is loaded. However, load is accounted for by using the channel occupancy figure, not by attempting to measure actual delays. The path bandwidth is simply the bandwidth in bits per second of the slowest link in the path. Channel occupancy indicates how much of that bandwidth is currently in use. It is measured, and will change with load. Reliability indicates the current error rate. It is the fraction of packets that arrive at the destination undamaged. It is measured.
Although they are not used as part of the metric, two addition pieces of information are passed with it: hop count and MTU. The hop count is simply the number of gateways that a packet will have to go through to get to the destination. MTU is the maximum packet size that can be sent along the entire path without fragmentation. (That is, it is the minimum of the MTUs of all the networks involved in the path.)
Based on the metric information, a single "composite metric" is calculated for the path. The composite metric combines the effect of the various metric components into a single number representing the "goodness" of that path. It is the composite metric that is actually used to decide on the best path.
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