From IGRP to EIGRP

The original motivation for developing EIGRP was simply to make IGRP classless. But early in the development the engineers working on the project recalled some academic proposals for a new kind of convergence algorithm and decided to use that algorithm in their extension of IGRP. The result was a protocol that, while retaining some concepts introduced with IGRP such as multiple metrics, protocol domains, and unequal-cost load balancing, is distinctly different from IGRP.

EIGRP is occasionally described as a distance vector protocol that acts like a link-state protocol. To recap the extensive discussion in Chapter 4, "Dynamic Routing Protocols," a distance vector protocol shares everything it knows, but only with directly connected neighbors. Link-state protocols announce information only about their directly connected links, but they share the information with all routers in their routing domain or area.

All the distance vector protocols discussed so far run some variant of the Bellman-Ford (or Ford-Fulkerson) algorithm. These protocols are prone to routing loops and counting to infinity. As a result, they must implement loop-avoidance measures such as split horizon, route poisoning, and hold-down timers. Because each router must run the routing algorithm on received routes before passing those routes along to its neighbors, larger networks might be slow to converge. More important, distance vector protocols advertise routes; the change of a critical link might mean the advertisement of many changed routes.

Compared to distance vector protocols, link-state protocols are far less susceptible to routing loops and bad routing information. The forwarding of link-state packets is not dependent on performing the route calculations first, so large networks might converge faster. And only links or prefixes and their states are advertised, not routes, which means the change of a link will not cause the advertisement of all routes using that link.

Regardless of whether other routing protocols perform route calculations before sending distance vector updates to neighbors or after building a topological database, their common denominator is that they perform the calculations individually. In contrast to the Bellman-Ford algorithms used by most other distance vector protocols, EIGRP uses a system of diffusing computationsroute calculations that are performed in a coordinated fashion among multiple routersto attain fast convergence while remaining loop-free at every instant.

Although EIGRP updates are still vectors of distances transmitted to directly connected neighbors, they are nonperiodic, partial, and bounded. Nonperiodic means that updates are not sent at regular intervals; rather, updates are sent only when a metric or topology change occurs. Partial means that the updates will include only routes that have changed, not every entry in the route table. Bounded means that the updates are sent only to affected routers. These characteristics mean that EIGRP uses much less bandwidth than typical distance vector protocols use. This feature can be especially important on low-bandwidth, high-cost Wide Area Network (WAN) links.

Another concern when routing over low-bandwidth WAN links is the maximum amount of bandwidth used during periods of convergence, when routing traffic is high. By default, EIGRP uses no more than 50 percent of the bandwidth of a link. Later IOS releases allow this percentage to be changed with the command ip bandwidth-percent eigrp.

EIGRP is a classless protocol (that is, each route entry in an update includes a subnet mask). Variable-length subnet masks may be used with EIGRP not only for sub-subnetting as described in Chapter 6, "RIPv2, RIPng, and Classless Routing," but also for address aggregationthe summarization of a group of major network addresses.

EIGRP packets can be authenticated using an MD5 cryptographic checksum. The basics of authentication and MD5 are covered in Chapter 6; an example of configuring EIGRP authentication is included in this chapter.

Finally, a major feature of EIGRP is that it can route not only IP but also IPX and AppleTalk.