What is EIGRP ?? | EIGRP Explained

Enhanced Interface Gateway Routing Protocol  (EIGRP)

Enhanced Interface Gateway Routing Protocol (EIGRP) EIGRP Explained is an advanced range vector routing protocol. This protocol is the development of an earlier Cisco protocol called IGRP, which is now considered outdated. EIGRP supports classless routing and VLSM, route synthesis, incremental updates, load balancing and many other useful features. This is a Cisco proprietary protocol. All routers on a network running the EIGRP protocol must be Cisco routers.

Routers who run EIGRP must be neighbors before changing route information. To dynamically identify neighbors, EIGRP routers use the multicast address. Each EIGRP router stores routing and topology information in three tables:

The neighboring table - stores information about neighbors in the EIGRP

Topology table - stores routing information acquired by nearby routers

Routing Table - stores the best roads

The administrative distance to the EIGRP protocol is 90, which is less than both the administrative distance from the RIP protocol and the administrative distance to the OSPF protocol. As a result, EIGRP stretches will be preferred over these roads. EIGRP uses a trusted transport protocol (RTP) to send messages.

EIGRP calculates its metrics by bandwidth, delay, reliability and load. By default, only bandwidth and delay in calculation are used, while reliability and load are set to zero.

EIGPR uses the concept of autonomous systems. An autonomous system is a set of EIGRP-enabled routers that will be EIGRP neighbours. Each router in an autonomous system must have the same autonomous system number configured, otherwise the routers will not be neighbours.

Neighbours EIGRP

The EIGRP protocol must establish neighbouring relationships with other routers adjacent to the EIGRP protocol prior to exchange of routing information. To create neighbouring relations, the routers send hello packages every couple of seconds. Hello package is sent to the multicast address at

On LAN Interface, Hellos is sent every 5 seconds. On WAN Interface every 60 seconds.

The following fields in a hatchet must be identical to the routers to be neighbours:

ASN (Autonomous System Number)
subset number
K-values ​​(components in metric)

Routers send hello packages every two seconds to ensure that the neighbourhood relationship is still active. By default, routers consider the neighbour to be in use after the end of a waiting hour. By default, the retention timer is three times the hay interval. On LAN, waiting time is 15 seconds.

Feasible and reported distance

Two terms that you will often encounter when working with EIGRP are achievable and reported distances. Clarify these terms:

Realistic Distance (FD) - Metric of the best way to reach a network. This route will be entered in the route table.

Declared Distance (RD): Metric published by a nearby router for a specific route. In other words, the metric route that the nearby router uses to reach the network.

To better understand the concept, consider the following examples.

reported achievable distance

The EIGRP protocol has been configured on R1 and R2. R2 is directly connected to the subnet and announces this subnet in the EIGRP protocol. Assume that metric R2 to reach this subnet is 28160. When the subnet is advertised to R1, R2 informs its metric to reach is 10. From R1, this metric is considered to be the distance reported for this route. R1 receives the update and adds the metrology to the neighbor at the reported distance. This measured value is called the achievable distance and is stored in the routing table of R1 (30720 in our case).

The possible and specified distances are shown in the EIGRP topology table of R1:

to display the topology feature of IP

Successor and successor achievable

Two other terms that often occur in the EIGRP world are successors and potential successors. A successor is the road with the best metric to reach a destination. This route is stored in the route table. Any successor is a backup path to reach the same destination, useful immediately if the tracker's route fails. These backup boxes are stored in the topology table.

In order for a route to be chosen as a possible successor, a condition must be met:

The advertised route (AD) for the neighbour of the route must be less than the successor's distance (FD).

The following example explains the term successor and achievable successor.

successors and successors that can be achieved.

EIGRP Explained Summary 

Here is a list of the most important features of the EIGRP:

*Advanced distance vector routing protocol

*Classless routing protocols

*Supports VLSM (variable length subnet mask)

*converged quickly

*Supports multiple protocols for network storage (IPv4, IPv6, IPX, AppleTalk ...)

*Uses the multicast address of for routing updates

*Sends partial routing updates

*Supports load balancing at equal and uneven costs

*Supports manual synthesis on any router within a network

*By default, bandwidth and delay use to calculate its measurement value
  Cisco property

*Supports MD5 authentication

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