Operation of EIGRP

EIGRP is a revised and improved version of IGRP. Its goal is to solve the scaling limitations that IGRP faces, using the distance vector technology from which it grew. EIGRP increases the potential growth of a network by reducing the convergence time. This is achieved by the following:

• The Diffusing Update Algorithm (DUAL)

• Loop-free networks

• Incremental updates

• Multicast addressing for updates

• Holding information about neighbors as opposed to the entire network

These features depend on proprietary technology, which centers on local computation. The DUAL algorithm diffuses this computation over multiple routers, with each router responsible for its own small calculation and making requests of neighboring routers when necessary. A full understanding of the concepts and operation of EIGRP will aid you in the design, implementation, and maintenance of EIGRP networks, and will definitely help you pass an exam on the subject.

The main concepts of EIGRP are as follows:

• Advanced distance vector

• Loop-free routing tables

• Support for different topologies

• Rapid convergence

• Reduced bandwidth use

• Use of a composite metric (bandwidth and delay as the default)

• Unequal load balancing

• Neighbor discovery

• Successors, the selection process for a feasible successor

• Passive and active routes

• Protocol independence at Layer 3, allowing support for IP, AppleTalk, and IPX

• Reliable sending of routing updates

• VLSM, which allows efficient addressing, discontiguous networks, and the use of classless networks

• Manual summarization

• Compatibility with IGRP

• Easy-to-configure nature

• Fewer design constraints than OSPF

How EIGRP Works

Cisco identifies four main components of EIGRP:

• Protocol-dependent modules

• Reliable Transport Protocol

• Neighbor discovery/recovery

These components are discussed in this section. However, to understand how EIGRP works, there must be some familiarity with the terminology. Table 7-2 defines the main concepts and gives a brief synopsis of each term.

Table 7-2 Terminology for EIGRP for IP




A router running EIGRP that is directly connected.

Neighbor table

A list of every neighbor, including the IP address, the outgoing

interface, the holdtime, SRTT, and uptime or how long since

the neighbor was added to the table. This table is built from

information on hellos received from adjacent routers


Route table

The routing table, or list of available networks and the best

paths. A path is moved from the topology table to the routing

table when a feasible successor is identified.

Topology table

A table that contains all the paths advertised by neighbors to

all the known networks. This is a list of all the successors,

feasible successors, the feasible distance, the advertised

distance, and the outgoing interface. DUAL acts on the topology

table to determine successors and feasible successors by which

to build a routing table.


Used to find and maintain neighbors in the topology table.

They are sent periodically and are sent reliably.


An EIGRP packet containing change information about the

network. It is sent reliably. It is sent only when there is a

change in the network to affected routers:

When a neighbor first comes up

When a neighbor transitions from active to passive for a


When there is a metric change for a destination


Sent from the router when it loses a path to a network. If there

is no alternate route (feasible successor), it will send out

queries to neighbors inquiring whether they have a feasible

successor. This makes the route state change to active. The

queries are sent reliably.


A response to the query. If a router has no information to send

in a reply, it will send queries to all its neighbors. A unicast is

sent reliably.


A hello packet with no data that is an acknowledgment of

packets sent reliably.


Sent in the hello packet. It determines how long the router

waits for hellos from a neighbor before declaring it unavailable.

This information is held in the neighbor table.

Table 7-2 Terminology for EIGRP for IP (Continued)



Smooth Round Trip Time (SRTT)

The time that the router waits after sending a packet reliably to

hear an acknowledgment. This is held in the neighbor table

and is used to calculate the RTO.

Retransmission Timeout (RTO)

Timer calculated in reference to the SRTT. RTO determines

how long the router waits for an ACK before retransmitting the


Reliable Transport Protocol (RTP)

Requirement that the packets be delivered in sequence and


Diffusing Update Algorithm (DUAL)

An algorithm performed on the topology table to converge the

network. It is based on a router detecting a network change

within a finite time, with the change being sent reliably and in

sequence. As the algorithm is calculated simultaneously, in

order and within a finite time frame on all affected routers, it

ensures a loop-free network.

Advertised distance (AD)

The cost of the path to the remote network from the neighbor

(the metric from the next-hop router).

Feasible distance (FD)

The lowest-cost distance (metric) to a remote network.

Feasible condition (FC)

When a neighbor reports a path (AD) that is lower than the

router's FD to a network.

The neighbor's (next-hop router's) path has a lower metric

than the router's path.

Feasible successor (FS)

The neighbor reporting the AD that is lower than the router's

FD becomes the feasible successor. The next-hop router that

meets the FC.


The next-hop router that passes the FC. It is chosen from the

FSs as having the lowest metric to the remote network.

Stuck in Active (SIA)

When a router has sent out network packets and is waiting

for ACKs from all its neighbors. The route is active until all the

ACKs have been received, if they do not appear after a certain

time, the router is Stuck in Active for the route.

Query scoping

Another term for SIA.


Route state when a network change is seen, but on

interrogation of the topology table, there is no FC. The router

queries its neighbors for alternative routes.


An operational route is passive. If the path is lost, the router

examines the topology table to find an FS. If there is an FS, it

is placed in the routing table and the router does not query the

others, which would send it into active mode.

Even if the computation of the network is local, the router must know about the entire network. The explanation of the routing protocol will be given through the viewpoint of one router. When the network communication between the routers running EIGRP is understood, the operation of EIGRP will become clear; the concepts and terms will be placed in context. This facilitates the memorization of the subject; rote learning is no longer necessary.

Was this article helpful?

+1 0

Post a comment