Appendix A. EIGRP for IP Basics of Operation

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

• Minimal use of network resources in normal operation?/span> EIGRP only transmits small hello packets during normal operation to maintain neighbor relationships; it has no periodic routing updates (flooding of the routing table to neighbors).

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

• Rapid convergence?/span> EIGRP converges quickly during topology changes, primarily because it computes all loop-free paths available to a given destination, rather than just the lowest-cost path (explained in more detail in the next section, "The Diffusing Update Algorithm (DUAL)").

• Scalability?/span> EIGRP can scale into large networks because no periodic updates are used and a minimal use of network resources during convergence exists.

A major revision of the protocol occurred in Cisco IOS Software Release 10.3(11), 11.0(8), and 11.1(3). The recommended practice is to run software implementing the later revision of EIGRP to promote stability and interoperability.

Cisco IOS Software Release 12.0(7)T introduced the stub feature, and IOS Software Release 12.1(5)T added Stuck in Active (SIA) enhancements. The stub feature was by far the most significant change to occur in the protocol to improve scalability, and the SIA enhancements dramatically improved the stability of EIGRP. Both features are extremely valuable capabilities and will be explained in further detail later in this appendix.

EIGRP is based on the Diffusing Update Algorithm (DUAL) to find the best loop-free paths through a network.