Appendix C. EIGRP Fundamentals

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector protocol with many advantages:

• Minimal use of network resources in normal operation— EIGRP transmits only small hello packets during normal operation to maintain neighbor relationships; there are no periodic routing updates (flooding of the routing table to neighbors).

• Restricted use of network resources when reacting to network topology changes— EIGRP transmits only information about what has changed and also restricts (paces) the rate at which it sends packets so that it will not overwhelm a link.

• Rapid convergence— EIGRP converges very quickly during topology changes.

• Scalability— Because there are no periodic updates, and there is minimal use of network resources during convergence, EIGRP can scale into very large networks.

A major revision of the protocol occurred in IOS revisions 10.3(11), 11.0(8), and 11.1(3). Running software that implements the later revision of EIGRP is recommended to promote stability and interoperability. The primary addition to EIGRP in the newer revision is the pacing of packets so that EIGRP won't use more than 50 percent of the available bandwidth; although there are others, this is the most important change. EIGRP is based on the Diffusing Update Algorithm (DUAL) to find the best loop-free paths through a network.

This appendix will give you a general overview of the protocol rather than a complete understanding of every aspect of EIGRP's operation.