EIGRP Enhance Interior Gateway Routing Protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) is a cisco proprietary Cisco protocol that runs on Cisco routers. Understanding EIGRP is very important because it is probably one of the two most popular routing protocols in used today. EIGRP is a classless, enhanced-vector protocol that gives us a real edge over another Cisco proprietary protocol, Interior Gateway Routing Protocol (IGRP). That’s basically why it’s call Enhanced IGRP, like IGRP, EIGRP uses the concept of an autonomous system to describe the set of contiguous routers that run the same routing protocol and share routing information. But unlike IGRP, EIGRP includes the subnet mask in its route updates. The advertisement of subnet information allows us to use Variable Length Subnet Mask (VLSM) and summarization when designing our networks!

           EIGRP is sometimes referred to as a hybrid routing protocol because it has characteristics of both distance-vector and link-state protocols. For example, EIGRP doesn’t send link-state packets as OSPF does; instead, it sends traditional distance-vector updates containing information about networks plus the cost of reaching them from the perspective of the advertising router. And EIGRP has link-state characteristics as well – it synchronizes routing tables between neighbors at startup and then sends specific updates only when topology changes occur. This makes EIGRP suitable for very large networks. EIGRP has a maximum hop count of 255 (the default is set to 100).

There are a number of powerful features that make EIGRP a real standout from IGRP and other protocols. The main features are:

·         Support for IP and IPv6 (and some other useless routed protocols) via protocol dependent modules.

·         Considered classless (same as RIPv2 and OSPF).

·         Support for VLSM/CIDR.

·         Best path selection via Diffusion Update Algorithm (DUAL).

·         Support for summaries and discontiguous networks.

·         Efficient neighbor discovery.

·         Communication via Reliable Transport Protocol (RTP).