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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  Other Interior Routing Protocols

The HELLO Protocol (HELLO) 1 2 Enhanced Interior Gateway Routing Protocol (EIGRP)

Features and Capabilities

An important difference between RIP and IGRP, however, is that where RIP only allows the cost to reach a network to be expressed in terms of hop count, IGRP provides a much more sophisticated metric. In IGRP, the overall cost to reach a network is computed based on several individual metrics, including internetwork delay, bandwidth, reliability and load. The calculation of cost can be customized by an administrator, who can set relative weightings to the component metrics to reflect the priorities of that autonomous system. So, if a particular administrator feels route cost would be best minimized by emphasizing reliability over bandwidth, he or she can do this. Such a system provides tremendous flexibility over the rigid hop-count system of RIP. Unlike RIP, IGRP also does not have any inherent limit of 15 hops between networks.

To this basic algorithm, IGRP adds a feature called multipath routing. This allows multiple paths between routes to be used automatically, with traffic shared between them. The traffic can either be shared evenly, or apportioned unevenly based on the relative cost metric of each path. This provides improved performance and again, flexibility.

Since IGRP is a distance-vector protocol like RIP, it shares many of RIP's algorithmic “issues”. Unsurprisingly, then, IGRP must incorporate many of the same stability features as RIP, including the use of split horizon, split horizon with poisoned reverse (in certain circumstances) and the employment of hold-down timers. Like RIP, IGRP also uses timers to control how often updates are sent, how long routers are “held down”, and how long routes are held in the routing table before being expired.

Cisco originally developed IGRP for IP networks, and since IP is predominant in the industry, this is where it is most often seen. IGRP is not specific to IP, however, and can be used with other internetworking protocols if implemented for them. As we will see in the next topic, Cisco also used IGRP as the basis for an improved routing protocol called EIGRP, developed several years after the original.

The HELLO Protocol (HELLO) 1 2 Enhanced Interior Gateway Routing Protocol (EIGRP)