EIGRP - Enhanced Interior Gateway Routing Protocol

Enhanced Interior Gateway Routing Protocol is a Cisco proprietary, classless, Interior gateway routing protocol that runs only on Cisco devices. It is an enhanced version of the Cisco's Interior Gateway Protocol (IGP), that's why the name "Enhanced IGRP".

EIGRP is sometimes referred to as a hybrid protocol, as it has characteristics of the distance vector as well as the link-state protocols. EIGRP no only supports IP or IPv6, but other protocols of the network layer: IPX, AppleTalk and protocol of Novell NetWare networks.

Like IGRP, EIGRP uses bandwidth, delay, load, reliability and MTU as its comparison metrics to calculate the best path. As the metrics for calculation is same for both IGRP and EIGRP routers using these protocols can easily interoperate with each other.

How does EIGRP works
In EIGRP, each router keeps a copy of its neighbours' routing table. If the destination node is not found in any of these tables, the router queries its neighbors for a route and they in turn query their neighbors until a route is found.

When a routing table entry changes in any of the router, it notifies its neighbor of this change only. Some earlier routing protocols sends the entire routing table on the change of even one entry. This improves overall network efficiency.

Along with that, each router sends the periodic "hello" packet to know the state of the neighbouring routers. It no "hello" message has been received for a certain period of time, the router is assumed to be inoperative.