Interior Gateway Routing Protocol (IGRP)
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If you have read a substantial portion of this Guide already, you have probably noticed that I greatly prefer universal, open standards to proprietary standards. (I explain the reasons why in the Networking Fundamentals section on standards.) I am far from alone in this view, and in fact, it's no exaggeration to say that much of the success of TCP/IP and the Internet is tied to the fact that they were both developed using the open RFC process, and still are to this day.
That said, there are certain situations where a proprietary protocol can be of benefit, and can even achieve considerable success, if a minimum of two factors are true. First, there must be either a lack of a suitable open protocol, or a gap in the feature coverage of existing open protocols that creates an opportunity for a proprietary protocol to succeed. Second, the proprietary protocol must be either initiated or strongly supported by a big player in the industry, to help ensure that other companies will take notice and give the protocol a chance to become a standard.
This situation arose in the 1980s in the world of routing protocols. At that time, the most popular interior routing protocol was the Routing Information Protocol (RIP). As described in the RIP section of this Guide, RIP does a basically good job, but has a number of limitations and problems that are inherent to the protocol and not easily resolved. In the mid-1980s, open alternatives like OSPF did not yet exist; even if it had, OSPF is much more complex than RIP and therefore sometimes not a good alternative.
Cisco Systems, definitely one of the big names in networking and especially internetworking and routing, decided to develop a new routing protocol that would be similar to RIP but would provide greater functionality and solve some of RIP's inherent problems: the Interior Gateway Routing Protocol (IGRP). IGRPwhich conveniently uses both the words gateway and routing in its name to convey the equivalence of the two words in internetworking standardswas designed specifically to be a replacement for RIP. It is similar in many ways, and keeps RIP's simplicity, one of its key strengths. At the same time, IGRP overcomes two key limitations of RIP: the use of only hop count as a routing metric, and the hop count limit of 15.
Like RIP, IGRP is a distance-vector routing protocol designed for use with an autonomous system, and thus uses the same basic mechanism for route determination. Each router routinely sends out on each local network to which it is attached a message containing a copy of its routing table. This message contains pairs of reachable networks and costs (metrics) to reach each network. A router receiving this message knows it can reach all the networks in the message as long as it can reach the router that sent the message. It computes the cost to reach those networks by adding to their costs, the cost to reach the router that sent the message. The routers update their tables accordingly, and send this information out in their next routine update. Eventually, each router in the autonomous system has information about the cost to reach each network in it.
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