What is EIGRP

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What is EIGRP?

� EIGRP is an advanced distance-vector routing protocol that relies on featurescommonly associated with link-stateprotocols.

� EIGRP uses Link State's partial updatesand neighbor discovery.

� EIGRP's advanced features supports IP,IPX and AppleTalk.

� EIGRP uses RTP (Reliable TransportProtocol) to transport its routing updates

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IGRP and EIGRP Metric Calculation - 1

� The composite metric is calculated with the

following formula:

� By default, k1=k3=1 and k2=k4=k5=0. The

default composite metric for EIGRP, adjusted

for scaling factors, is as follows:

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EIGRP Neighbor Discovery -1

� EIGRP routers actively establish relationships with their

neighbors, similar to what Link State routers do.� EIGRP routers establish adjacencies with neighbor routers by

using small hello packets.

� TheHello protocol uses a multicast address of 224.0.0.10,and all routers periodically send hellos.

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EIGRP Neighbor Discovery - 2

� On hearing hellos, the router creates a table of itsneighbors.

� The continued receipt of these packets maintainsthe neighbor table

� To become a neighbor, the following conditions mustbe met:

± The router must hear a hello packet or an ACK from aneighbor.

± The AS number in the packet header must be the same asthat of the receiving router.

± The neighbor¶s metric settings must be the same.

± Each Layer 3 protocol has its own neighbor table.

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Neighbor Discovery - 3

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Neighbor Discovery - 4

By forming adjacencies, EIGRP routers do the

following:

� Dynamically learn of new routes that jointheir network

� Identify routers that become either

unreachable or inoperable� Rediscover routers that had previously

been unreachable

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EIGRP Neighbor Tables

� The most important table in EIGRP is the neighbor table

and relationships tracked in the neighbor table are the

basis for all the EIGRP routing update and convergence

activity.

� The neighbor table contains information about

adjacent neighboring EIGRP routers.

� A neighbor table is used to support reliable, sequenced

delivery of packets.� An EIGRP router can maintain multiple neighbor tables,

one for each PDM running (e.g., IP, IPX, and AppleTalk)

routed protocols.

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EIGRP Packet Types - 1

� Hello packets assist in the discovery of EIGRP

neighbors. The packets are multicast to 224.0.0.10.

� An acknowledgment packet acknowledges the

reception of an update packet. An acknowledgment

packet is a hello packet with no data.

Acknowledgment packets are sent to the unicast

address of the sender of the update packet.

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EIGRP Topology Table - 1

� EIGRP uses its topology table to store all the information it

needs to calculate a set of distances and vectors to all

reachable destinations.

� EIGRP maintains a separate topology table for each routed

protocol.

� EIGRP sorts the topology table so that the successor routes

are at the top, followed by feasible successors.

� Entries in the topology table can be in one of two states:

active or passive.

� A passive route is one that is stable and available for use. An

active route is a route in the process of being recomputed by

DUAL.

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EIGRP Routing Table

� The routing table contains the routes installed by DUALas the best loop-free paths to a given destination

� EIGRP will maintain up to four routes per destination.These routes can be of equal or unequal cost.

�EIGRP routers maintain a separate routing table for eachrouted protocol.

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