How EIGRP Routing Works

EIGRP operates through the use of four key technologies:

Neighbor Discovery. Similar to link state protocols, EIGRP routers also periodically send out “hello” packets, letting neighboring routers know that they are functioning and available. On a LAN or point-to-point links, these message are sent out as multicasts every 5 seconds. On a multipoint network (like Frame Relay) with speeds lower than T1, these packets are unicast every 60 seconds. As long as these “hello” packets are received, an EIGRP router assumes that its neighbors are available for the purpose of exchanging routing table information. If three “hello” periods pass without receiving a “hello” message, a router will consider its neighbor unavailable and make the necessary routing table changes. On a LAN, this can happen in as little as 15 seconds (3 times the “hello” message interval).

Reliable Transport Protocol. The Reliable Transport Protocol is responsible for ensuring that EIGRP updates actually reach neighboring routers, in the correct order. EIGRP updates are sent out as multicasts to address When a neighboring router receives an update, RTP requires that an acknowledgement be sent. This is different than many routing protocols, which send update traffic in a connectionless manner.

Diffusing Update Algorithm. DUAL is the protocol used by EIGRP to ensure fast convergence and that the most efficient loop-free route advertised by neighbors is the one added to a router’s routing table. DUAL uses the lowest calculated metric to determine the best path to a destination, referred to as the feasible distance. Routers that advertise a lower metric to the destination than the feasible distance are known as feasible successors, and are ultimately used as the next hop router to which packets will be sent. When a topology change occurs, an EIGRP router will use the route provided by the next most feasible successor as the next hop. In cases where all metrics are higher than the feasible distance, the EIGRP router must recompute the route.

Protocol-Dependent Modules. Because it is capable of routing multiple protocols (IP, IPX, and AppleTalk), EIGRP implements what are known as protocol-dependent modules. For example, the IP EIGRP module will automatically redistribute IGRP routes into EIGRP and vice versa. Similarly, AppleTalk EIGRP will redistribute routes into and out of AppleTalk RTMP.

EIGRP offers greater flexibility, reliability, and better convergence times than a traditional distance-vector protocol. One limiting factor is that EIGRP is proprietary to Cisco – as such, EIGRP is limited to networks running Cisco equipment.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.