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EIGRP Protocol

Enhanced IGRP (EIGRP):-is a classless, enhanced distance-vector protocol that gives us a real edge over another Cisco proprietary protocol, EIGRP uses the concept of an autonomous system to describe the set of contiguous routers that run the same routing protocol and share routing information. EIGRP is sometimes referred to as a hybrid routing protocol because it has characteristics of both distance-vector and link-state protocols. EIGRP has a maximum hop count of 255 (the default is set to 100).

There are a number of powerful features that make EIGRP a real standout from IGRP and other protocols. The main ones are listed here:
  • Support for IP and IPv6 (and some other useless routed protocols) via protocol-dependent modules
  • Considered classless (same as RIPv2 and OSPF)
  • Support for VLSM/CIDR
  • Support for summaries and discontiguous networks
  • Efficient neighbor discovery
  • Communication via Reliable Transport Protocol (RTP)
  • Best path selection via Diffusing Update Algorithm (DUAL)
Neighbor Discovery

Before EIGRP routers are willing to exchange routes with each other, they must become neighbors.

There are three conditions that must be met for neighborship establishment:
  • Hello or ACK received
  • AS numbers match
  • Identical metrics (K values)
Neighbor table

Each router keeps state information about adjacent neighbors. When a newly discovered neighbor is learned, the address and interface of the neighbor are recorded,and this information is held in the neighbor table, stored in RAM. There is one neighbor table for each protocol-dependent module.

Topology table

It contains all destinations advertised by neighboring routers, holding each destination address and a list of neighbors that have advertised the destination.

Feasible distance

This is the best metric along all paths to a remote network, including the metric to the neighbor that is advertising that remote network. This is the route that you will find in the routing table because it is considered the best path.

Reported/advertised distance

This is the metric of a remote network, as reported by a neighbor.It is also the routing table metric of the neighbor and is the same as the second number in parentheses as displayed in the topology table, the first number being the feasible distance.

Feasible successor

A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. EIGRP will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor) is copied and placed in the routing table. The show ip eigrp topology command will display all the EIGRP feasible successor routes known to a router.


A successor to neighbor route having least cost path towards the destination.

Reliable Transport Protocol (RTP)

EIGRP uses a proprietary protocol called Reliable Transport Protocol (RTP)to manage the communication of messages between EIGRP-speaking routers. And as the name suggests,reliability is a key concern of this protocol. Cisco has designed a mechanism that leverages multicasts and unicasts to deliver updates quickly and to track the receipt of the data. When EIGRP sends multicast traffic, it uses the Class D address

Diffusing Update Algorithm (DUAL)

EIGRP uses Diffusing Update Algorithm (DUAL)for selecting and maintaining the best pathto each remote network. This algorithm allows for the following:
  • Backup route determination if one is available
  • Support of VLSMs
  • Dynamic route recoveriesM
  • Queries for an alternate route if no route can be found
Neighborship table

The neighborship table(usually referred to as the neighbor table) records information about routers with whom neighborship relationships have been formed.

Topology table

The topology table stores the route advertisements about every route in the internetwork received from each neighbor.

Route table

The route table stores the routes that are currently used to make routing decisions. There would be separate copies of each of these tables for each protocol that is actively being supported by EIGRP, whether it’s IP or IPv6.

EIGRP Metrics
  • Bandwidth
  • Delay
  • Load
  • Reliability
  • maximum transmission unit (MTU)

R1#show run | sec router eigrp

routereigrp 1
no auto-summary

R2#show run | sec router eigrp

routereigrp 1
no auto-summary

R3#show run | sec router eigrp routereigrp 1

no auto-summary

R1#sh ip route eigrp is subnetted, 1 subnets
D [90/156160] via, 00:01:19, FastEthernet0/0

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