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CISCO IOS in a Nutshell by James Boney

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IGRP

The Interior Gateway Routing Protocol (IGRP) is a distance-vector routing protocol. IGRP has been superseded by Enhanced IGRP (EIGRP), which has many new features and is covered later in this chapter. The two protocols are fundamentally similar, configuration-wise, and this section serves as an introduction for both.

IGRP and EIGRP have a compound metric that takes into account several factors, such as link bandwidth and latency. As such, IGRP is superior to RIP, which takes into account only the hop count, and RIPv2, which uses both hop count and bandwidth. In addition to the compound metric, which allows better route selection, IGRP tends to have better convergence times, meaning that routing stabilizes more quickly after a network disruption. In addition, although it is more difficult to configure than RIP, configuration is still relatively easy.

The biggest drawback of IGRP (and EIGRP) is that it’s a proprietary protocol, which means it is implemented only by Cisco routers. If all you have on your network are Cisco devices, using IGRP is not a problem. However, if you have a multivendor environment, you’ll be forced to use multiple routing protocols or to agree on a protocol (such as RIP or OSPF) that is supported by all your vendors. Another disadvantage is that IGRP (like RIP) broadcasts the entire routing table, which can consume a lot of network bandwidth. In addition, IGRP does not support VLSM (again, like RIP). If you use VLSM—and you probably should—you need to use ...

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