While IGRP might be a better solution than RIP when it comes to scalability, EIGRP takes things many steps further. First of all, EIGRP is classless, meaning that it supports the use of VLSM. Unlike IGRP, EIGRP supports the routing of multiple protocols, including IP, AppleTalk, and IPX. EIGRP is usually described as a hybrid protocol, meaning that it displays characteristics of both a distance vector and link state protocol.
EIGRP uses the same metrics as IGRP in making its routing decisions – bandwidth, delay, reliability, load, and MTU. The default metrics used are again the same, bandwidth and delay. However, for a more granular level of control, EIGRP multiplies each of the metrics by 256 before performing the calculation of the composite metric. EIGRP was designed to make much better use of bandwidth, and to allow routers to have a much better awareness of neighboring routers.
Instead of sending its entire routing table out at regular intervals, an EIGRP router instead sends out only partial updates, and even then, only when a route changes. Obviously this makes better use of the available network bandwidth. An EIGRP router also has a more complete view of the network than a typical distance vector protocol – it not only maintains its own routing table, but also keeps a copy of the routing tables of neighboring routers. When an EIGRP router cannot find a route to a network based on all the information it currently has, it sends out a query to other routers, which is propagated until a route is found.
