Overview
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol based on IGRP. Unlike IGRP, which is a classful routing protocol, EIGRP supports CIDR, allowing network designers to maximize address space by using CIDR and VLSM. Compared to IGRP, EIGRP boasts faster convergence times, improved scalability, and superior handling of routing loops. Furthermore, EIGRP can replace Novell RIP and AppleTalk Routing Table Maintenance Protocol (RTMP), serving both IPX and AppleTalk networks with powerful efficiency.

You may have heard EIGRP described as a hybrid routing protocol offering the best of distance-vector and link-state algorithms. Technically, EIGRP is an advanced distance-vector routing protocol that relies on features commonly associated with link-state protocols. Some of OSPF's best traits, such as partial updates and neighbor discovery, are similarly put to use by EIGRP. However, OSPF's benefits, especially its hierarchical design, come at a price: administrative complexity. As seen in Chapter 5, Multiarea OSPF, multiarea implementation of OSPF requires mastery of a complex terminology and command set. On the other hand, EIGRP's advanced features can be easily implemented and maintained. Although it does not mirror OSPF's classic hierarchical design, EIGRP is an ideal choice for large, multiprotocol networks built primarily on Cisco routers.

This chapter surveys EIGRP's key concepts, technologies, and data structures. This conceptual overview is then followed by a study of EIGRP convergence and basic operation. Finally, this chapter shows how to configure and verify EIGRP, including using route summarization.