EIGRP : The complete guide for Beginner and Expert

In this article, we will discuss EIGRP (Enhanced Interior Gateway Routing Protocol), which is a hybrid routing protocol. It was originally defined by Cisco and available on only Cisco devices. Since 2013, it’s now an Open Standard and defined in RFC 7868. EIGRP works on Network Layer of OSI Model and protocol number is 88.

introduction-to-eigrp

 

Quick Overview of EIGRP

EIGRP stands for Enhanced Interior Gateway Routing Protocol. In 1993, IGRP (Interior Gateway Routing Protocol), is replaced by EIGRP, because IGRP doesn’t support IPv4 classes routing. EIGRP is the combination of Distance Vector (RIP) and Link State (OSPF) routing protocols. Thus, EIGRP protocol known as Advance Distance Vector or Hybrid routing protocol.

EIGRP is a dynamic routing protocol, which is used to share routes/ prefix information to neighboring routers. Unlike RIP, EIGRP only sends triggered updates to neighbor routers.

Different types of Tables used in EIGRP

 

Unlike RIP, EIGRP has Neighbor Table and Topology Table in addition to the Routing Table. We will discuss all these tables one by one.

different-eigrp-tables

Routing Table

The purpose of the EIGRP Routing Table is the same as other routing protocols. A router makes a forwarding decision on the basis of the Routing Table.  The best route from the topology table is transferred in the Routing Table.

Topology Table

Topology table has various routes which are learned from different neighbor routers. This contains detailed information about the route’s like Metric, Destination. Unlike routing table, route of the topology table never participated in routing. However, the Topology Table has backup routes. If the router lost their best route from their routing table, then the topology table provides the backup path as the best path to the routing table.

The best path of Topology Table is used in the Routing Table.

Neighbor Table

Neighbor Table, as the name suggests, have detailed information about neighbor routers which are directly connected. However, it does not contain any information about indirectly connected routers.

EIGRP Metric Calculation

Unlike RIP, the metric of any route in EIGRP depends upon any parameters. In fact, EIGRP uses 6 different parameters in order to calculate the matrix. These parameters are Bandwidth, Load, Total Delay, Reliability, MTU and Hop Count. By default, minimum Bandwidth and Total Delay is used for Metric calculation. But, an administrator can use all these parameters in order to calculate the route’s metric. These parameters are known as K-Values.

Features of EIGRP Protocol

EIGRP uses the Diffusing Update Algorithm (DUAL), which helps to calculate the best feasible successor and also prevent routing loops. It supports both Classless Inter-Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM). It can support Unequal Cost Load Balancing with the help of variance command. We can also apply authentication for security reasons. Unlike RIP, sends only triggered update, not full update.

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Summary

In this article, we discussed the Enhanced Interior Gateway Routing Protocol (EIGRP). There are three different tables used in EIGRP. All routes are present in the Topology Table. However, the best routes of the Topology Table are copied into the Routing Table. For reliability purpose, it uses RTP (Real-Time Trunking Protocol).

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