Balanced Hybrid Protocols Enhanced IGRP

EIGRP does not use distance vector or link-state logic, but instead it uses a whole new category of routing protocol. This new category has some features similar to link-state protocols, others similar to distance vector protocols, and yet others unlike either of the two. Cisco sometimes categorizes EIGRP as a balanced hybrid protocol, so you should remember the term.

The internal workings of EIGRP depend on an algorithm called the Diffusing Update Algorithm (DUAL). DUAL exchanges more topology information than a distance vector routing protocol, but it does not transmit full topology information like a link-state protocol. Also, the computations used by DUAL require far less processing than the computation-intensive Dijkstra SPF algorithm.

DUAL defines a method for each router not only to calculate the best current route to each subnet, but also to calculate alternative routes that could be used if the current route fails. An alternative route, using what DUAL calls a feasible successor route, is guaranteed to be loop-free. So, if the current best route fails, the router immediately can start using the feasible successor route instead so that convergence can happen very quickly.

The following list points out some of the key similarities to some of the other protocols covered in this introduction:

■ Like OSPF and Integrated IS-IS, it converges quickly, often in less than 3 seconds after a failure is recognized.

■ Like OSPF, EIGRP discovers neighbors before sending them routing information.

■ Like RIP and IGRP, EIGRP requires very little design effort. (Link-state protocols require some design work in medium to larger networks).

■ Like IGRP, EIGRP is Cisco proprietary.

■ Like IGRP, EIGRP uses a metric based on bandwidth and delay. EIGRP uses the same metric as IGRP, except that EIGRP scales the metric by multiplying by 256.

■ Like link-state protocols, EIGRP does not send full updates on a periodic interval, but rather sends partial updates only as links or routers go up and down.

■ Like link-state protocols, EIGRP builds some topology tables in addition to the IP routing table.

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