Enhanced Interior Gateway Routing Protocol (EIGRP) is an Open standard routing protocol loosely based on their original IGRP created by Cisco. Most of the routing optimizations are based on Diffuse Update Algorithm (DUAL), which guarantees loop-free operation and provide a mechanism for fast convergence. EIGRP has following features: Simple configuration Flexibility in summarization Unequal cost load balancing Scalability Authentication VLSM support Keeps Backup routes Incremental updates EIGRP is the only routing protocol which keeps backup routes in its topology table. So when the primary route fails, it pick that backup route from the topology table and puts it into the routing table, eliminating the need of querying a new route from neighbors on failure on primary link, which results in fast convergence.