Enhanced Interior Gateway Routing Protocol (EIGRP)
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Cisco Systems, a leader in the world of internetworking and routing technology, developed the Interior Gateway Routing Protocol (IGRP) in the mid-1980s as an alternative protocol to RIP for use in TCP/IP autonomous systems. IGRP represented a substantial improvement over RIP, but like any successful company, Cisco was not content to rest on its laurels. Cisco knew that IGRP had significant room for improvement, so they set to work on creating a better version of IGMP in the early 1990s. The result was the Enhanced Interior Gateway Routing Protocol (EIGRP).
Compared to the original protocol, EIGRP is more of an evolution than a revolution. EIGRP is still a distance-vector protocol, but is more sophisticated than other distance-vector protocols like IGRP or RIP, and includes certain features that are more often associated with link-state routing protocols like OSPF than distance-vector algorithms. Also, since Cisco realized many of the organizations deciding to use EIGRP would be migrating to it from IGRP, they took special steps to maximize compatibility between the two.
The chief differences between IGRP and EIGRP are not in what they do, but how they do it. EIGRP changes the way that routes are calculated, in an effort to improve efficiency and the speed of route convergence (agreement on routes between different routers in the internetwork). EIGRP is based on a new route calculation algorithm called the Diffusing Update Algorithm (DUAL), developed at a company called SRI International by Dr. J. J. Garcia-Luna-Aceves.
DUAL differs from a typical distance-vector algorithm primarily in that it maintains more topology information about the internetwork than that used by protocols like RIP or IGRP. It uses this information to automatically select least-cost, loop-free routes between networks. EIGRP uses a metric that combines an assessment of the bandwidth of a link and the total delay to send over the link. (Other metrics are configurable as well, though not recommended.) When a neighboring router sends changed metric information, routes are recalculated and updates sent as needed. DUAL will query neighboring routers for reachability information if needed (for example, if an existing route fails).
This as needed aspect of operation highlights an important way that EIGRP improves performance over IGRP. EIGRP does not send routine route updates, only partial updates as required, reducing the amount of traffic generated between routers. Furthermore, these updates are designed so that only the routers that need the updated information receive them.
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