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 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.

Routes are propagated in EIGRP in a distance vector manner, from neighbor to neighbor, and only the best routes are sent onward. A router that runs EIGRP does not have a complete view of a network because it sees only the routes it receives from its neighbors. In contrast, with a pure linkstate protocol (OSPF and IS-IS), all routers in the same area have identical information and therefore have a complete view of the area and its link states.

Recall that the default EIGRP metric calculation uses the minimum bandwidth and cumulative delay of the path. Other parameters can also be used in this calculation, including worst reliability between source and destination and worst loading on a link between source and destination. Note that the maximum transmission unit and hop count are carried in the EIGRP routing updates but are not used in the metric calculation.

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