As internetworks grew in scale and diversity in the early 1990s, new routing protocols were needed. Cisco developed Enhanced Interior Gateway Routing Protocol (IGRP) primarily to address many of the limitations of IGRP and RIP. As WANs were growing, so was the need for a routing protocol that would use efficient address space on WAN links, as well as the LAN networks. OSPF was available, but the CPU-intensive tasks that it had to perform often overloaded the small processors of many edge or remote routers of that time. The configuration was also more complex than that of RIP or IGRP. A routing protocol was needed that could support VLSM and that could scale with large internetworks, yet that was less CPU-intensive than OSPF. In 1994, Cisco answered the call by releasing Enhanced IGRP in Cisco IOS Software Release 9.21. Today, EIGRP is used as the routing protocol on many large government and commercial internetworks. It has proven to be very stable, flexible, and fast. In addition to these characteristics, the ease of EIGRP configuration makes it one of the most popular routing protocols among network engineers.
EIGRP can be referred to as a hybrid protocol. It combines most of the characteristics of traditional distance vector protocols with some characteristics of link-state protocols. Specifically, EIGRP is "enhanced" by using four routing technologies:
• Neighbor discovery/recovery
• Reliable Transport Protocol (RTP)
• DUAL finite-state machine
• Protocol-dependent modules
This chapter covers these technologies, as well as the operation and configuration of EIGRP.
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