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IGRP - Posted at 15:02 on 20/1/2007 by @lfons

Comparison with RIP

This section compares IGRP with RIP. This comparison is useful because RIP is used widely for purposes similar to IGRP. However, doing this is not entirely fair. RIP was not intended to meet all of the same goals as IGRP. RIP was intended for use in small networks with reasonably uniform technology. In such applications it is generally adequate.

The most basic difference between IGRP and RIP is the structure of their metrics. Unfortunately this is not a change that can simply be retrofitted to RIP. It requires the new algorithms and data structures present in IGRP.

RIP uses a simple "hop count" metric to describe the network. Unlike IGRP, where every path is described by a delay, bandwidth, etc., in RIP it is described by a number from 1 to 15. Normally this number is used to represent how many gateways the path goes through before getting to the destination. This means that no distinction is made between a slow serial line and an Ethernet. In some implementations of RIP, it is possible for the system administrator to specify that a given hop should be counted more than once. Slow networks can be represented by a large hop count. But since the maximum is 15, this can't be done very much. E.g. if an Ethernet is represented by 1 and a 56Kb line by 3, there can be at most 5 56Kb lines in a path, or the maximum of 15 is exceeded. In order to represent the full range of available network speeds, and allow for a large network, studies done by Cisco suggest that a 24-bit metric is needed. If the maximum metric is too small, the system administrator is presented with an unpleasant choice: either he can not distinguish between fast and slow routes, or he can't fit his whole network into the limit. In fact a number of national networks are now large enough that RIP can not handle them even if every hop is counted only once. RIP simply can't be used for such networks.

The obvious response would be to modify RIP to allow a larger metric. Unfortunately, this won't work. Like all distance vector protocols, RIP has the problem of "counting to infinity". This is described in more detail in RFC 1058 . When topology changes, spurious routes will be introduced. The metrics associated with these spurious routes slowly increase until they reach 15, at which point the routes are removed. 15 is a small enough maximum that this process will converge fairly quickly, assuming that triggered updates are used. If RIP were modified to allow a 24-bit metric, loops would persist long enough for the metric to be counted up to 2**24. This is not tolerable. IGRP has features designed to prevent spurious routes from being introduced. These are discussed below in section 5.2. It is not practical to handle complex networks without introducing such features or changing to a protocol such as SPF.


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