Enhanced Interior Gateway Routing Protocol (EIGRP) is an update to Cisco's now obsolete Interior Gateway Routing Protocol (IGRP). The original protocol was built to overcome issues in Routing Information Protocol (RIP). IGRP was still a distance vector routing protocol, but in addition to distance, Cisco allowed the protocol to carry more information about the path, such as bandwidth, delay, load, MTU, and reliability.
Cisco also increased the maximum hop count to 255 but allowed it to be configurable. Rather than broadcasting updates every 30 seconds, IGRP stretched the time out to 90 seconds, reducing load on the network but increasing convergence time. As with RIPv1, IGRP is a class-based routing protocol, and the data about the routes does not include subnet mask information.
With the advent of EIGRP, Cisco reduced the time to convergence by passing data to other routers only when neighboring routers change. If there is a new adjacent router, EIGRP will pass that information out to all of its routing partners or out to all of its network interfaces.
EIGRP stores its routing information in the router in three basic tables:
Neighbor table: This table stores the addresses of neighbors, those routers that are directly accessible through the routers own local interfaces. If the path to a targeted router has to go through another router, then that targeted router is not a neighbor.
Topology table: This table stores the routing tables which this router has received from neighboring routers. With this information, this router identifies the best route to each possible destination network, as well as identifying a successor and a feasible successor. The successors will be used when the primary route to the destination fails.
Routing table: This table is built from the topology table and contains just the routing information to each destination network. It includes successors as the primary route to the destination and feasible successors as backup routes where applicable and depending on the configuration of EIGRP.
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Source:http://www.dummies.com/how-to/content/enhanced-interior-gateway-routing-protocol-eigrp.html
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