EIGRP overview

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance vector routing protocol. It is sometimes referred to as a hybrid routing protocol because it has characteristics of both distance-vector and link-state routing protocols. EIGRP replaced Interior Gateway Routing Protocol (IGRP), an older proprietary Cisco routing protocol. EIGRP was also a proprietary protocol, but Cisco decided in 2013 to convert it to an open standard. This routing protocol is mostly used on Cisco devices and all routers in the network must support it.
The following features make EIGRP especially helpful in large and complex networks:

• support for classless routing and VLSM (Variable Length Subnet Masking).
• routes can be summarized on any router in the network.
• incremental routing updates.
• support for load-balancing.
• support for the MD5 authentication.
• support for IPv4 and IPv6.

EIGRP uses Cisco’s Reliable Transport Protocol (RTP) to send messages and uses the multicast address of 224.0.0.10. The default administrative distance of EIGRP is 90, which is less than the default administrative distances of RIP and OSPF, which means that EIGRP routes will be preferred over RIP and OSPF routes. The metric is calculated using bandwidth, delay, reliability and load. By default, only bandwidth and delay are considered when calulating metric, while reliability and load are set to zero.