- Posted at 15:00 on 20/1/2007 by @lfons
The IGRP protocol allows a number of gateways to coordinate their routing. Its goals are the following:
· Stable routing even in very large or complex networks. No routing loops should occur, even as transients.
· Fast response to changes in network topology.
· Low overhead. That is, IGRP itself should not use more bandwidth than what is actually needed for its task.
· Splitting traffic among several parallel routes when they are of roughly equal desirability.
· Taking into account error rates and level of traffic on different paths.
The current implementation of IGRP handles routing for TCP/IP. However, the basic design is intended to be able to handle a variety of protocols.
No one tool is going to solve all routing problems. Conventionally the routing problem is broken into several pieces. Protocols such as IGRP are called "internal gateway protocols" (IGPs). They are intended for use within a single set of networks, either under a single management or closely coordinated managements. Such sets of networks are connected by "external gateway protocols" (EGPs). An IGP is designed to keep track of a good deal of detail about network topology. Priority in designing an IGP is placed on producing optimal routes and responding quickly to changes. An EGP is intended to protect one system of networks against errors or intentional misrepresentation by other systems, BGP is one such Exterior gateway protocol.. Priority in designing an EGP is on stability and administrative controls. Often it is sufficient for an EGP to produce a reasonable route, rather than the optimal route.
IGRP has some similarities to older protocols such as Xerox's Routing Information Protocol,
Like these older protocols, IGRP is a distance vector protocol. In such a protocol, gateways exchange routing information only with adjacent gateways. This routing information contains a summary of information about the rest of the network. It can be shown mathematically that all of the gateways taken together are solving an optimization problem by what amounts to a distributed algorithm. Each gateway only needs to solve part of the problem, and it only has to receive a portion of the total data.
The major alternative to IGRP is Enhanced IGRP (EIGRP) and a class of algorithms referred to as SPF (shortest- path first). OSPF uses this concept. To learn more about OSPF refer to OSPF Design Guide. OSPF These are is based on a flooding technique, where every gateway is kept up to date about the status of every interface on every other gateway. Each gateway independently solves the optimization problem from its point of view using data for the entire network. There are advantages to each approach. In some circumstances SPF may be able to respond to changes more quickly. In order to prevent routing loops, IGRP has to ignore new data for a few minutes after certain kinds of changes. Because SPF has information directly from each gateway, it is able to avoid these routing loops. Thus it can act on new information immediately. However, SPF has to deal with substantially more data than IGRP, both in internal data structures and in messages between gateways.
IGRP is intended for use in gateways connecting several networks. We assume that the networks use packet-based technology. In effect the gateways act as packet switches. When a system connected to one network wants to send a packet to a system on a different network, it addresses the packet to a gateway. If the destination is on one of the networks connected to the gateway, the gateway will forward the packet to the destination. If the destination is more distant, the gateway will forward the packet to another gateway that is closer to the destination. Gateways use routing tables to help them decide what to do with packets. Here is a simple example routing table. (Addresses used in the examples are IP addresses taken from
network gateway interface
------- ------- ---------
128.6.4 none ethernet 0
128.6.5 none ethernet 1
128.6.21 18.104.22.168 ethernet 0
128.121 22.214.171.124 ethernet 1
10 126.96.36.199 ethernet 1
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