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8/7/2019 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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