EIGRP is a Cisco-proprietary protocol for routing IPv4; EIGRP can also be configured for routing IP version 6 (IPv6), Internetwork Packet Exchange (IPX), and AppleTalk traffic. EIGRP is an enhanced version of IGRP, which is a pure distance vector protocol. EIGRP, however, is a hybrid routing protocol—it is a distance vector protocol with additional link-state protocol features.

EIGRP features include the following:

■ Uses triggered updates (EIGRP has no periodic updates).

■ Uses a topology table to keep all routes received from its neighbors, not only the best routes.

■ Establishes adjacencies with neighboring routers using the Hello protocol.

■ Uses multicast, rather than broadcast, for communication.

■ Supports VLSM.

■ Supports manual route summarization. EIGRP summarizes on major network boundaries by default, but this feature can be turned off, and summarization can be configured at any point in the network.

■ Can be used to create hierarchically structured, large networks.

■ Supports unequal-cost load balancing.

EIGRP Characteristics

The characteristics that make EIGRP suitable for deployment in enterprise networks include the following:

■ Fast convergence: One advantage of EIGRP is its fast-converging DUAL route calculation mechanism. This mechanism allows backup routes (the feasible successors) to be kept in the topology table for use if the primary route fails. Because this process occurs locally on the router, the switchover to a backup route (if one exists) is immediate and does not involve action in any other routers.

■ Improved scalability: Along with fast convergence, the ability to manually summarize also improves scalability. EIGRP summarizes routes on classful network boundaries by default. Automatic summarization can be turned off, and manual summarization can be configured at any point in the network, improving scalability and network performance because the routing protocol uses fewer resources.

■ Use of VLSM: Because EIGRP is a classless routing protocol, it sends subnet mask information in its routing updates and therefore supports VLSM.

■ Reduced bandwidth usage: Because EIGRP does not send periodic routing updates as other distance vector protocols do, it uses less bandwidth—particularly in large networks that have a large number of routes. On the other hand, EIGRP uses the Hello protocol to establish and maintain adjacencies with its neighbors. If many neighbors are reachable over the same physical link, as might be the case in NBMA networks, the Hello protocol might create significant routing traffic overhead. Therefore, the network must be designed appropriately to take advantage of EIGRP’s benefits.

■ Multiple network layer protocol support: EIGRP supports multiple network layer protocols through Protocol-Dependent Modules (PDM). PDMs include support for IPv4, IPv6, IPX, and AppleTalk.