Enhanced Interior Gateway Routing Protocol

EIGRP is the best routing protocol as it supports many IP services, it supports IPv4 and IPv6. The protocol is very fast consumes less resources. The only con of this protocol is that, it is Cisco proprietary you have to have Cisco devices to run this protocol, where OSPF is open standard and supported by all other devices as well.
This protocol is version two the first version was IGRP. The beauty of this protocol is that its routing engine was designed first and then different modules were attached with it. Now if there is any need to introduce a new feature in this protocol the engineers will not redesign it, instead they will create a new module for feature and that will be attached with router.

EIGRP is a distance-vector routing protocol and it is highly efficient in many things.


Smart Database

EIGRP uses Distance-vector algorithm and it does not makes map of entire network and stores in its database. It only knows about its neighbor router and knows well about neighbor.

Smart Decision

EIGRP uses bandwidth as its primary metric. However, it has 5 different metrics.

OSPF uses cost as its metric and we saw it had problem when the actual bandwidth gets higher and higher as shown in figure below.


OSPF rounds the digit and this ends up calculating lower cost no matter how higher to bandwidth is. It is same for 1Gb, 10Gb, 100GB, and any higher number. This problem can be solved with some extra working but by default this protocol has this problem.

EIGRP makes a smart decision in this sense because it multiplies the bandwidth with 256. The formula is as below:

Bandwidth = (Reference Bandwidth / Actual Bandwidth) * 256.


EIGRP uses 5 different parameters for its metric, from 5 parameters one has been permanently turned off and you can do nothing about it and it is MTU.

The algorithm for this metric is called Difusing Update Algorithm Finite-State Machine. This algorithm holds 5 parameters and 2 parameters are always on by default.


Bandwidth and Delay are primary parameters of algorithm and are always turned on to calculate the metric. Bandwidth is the speed of connection and Delay is the time that is required with the bandwidth to send the data.

Reliability checks the reliability of connection (media) and it is slower to calculate change in the connection. This parameters uses more resources and is not reliable in detecting changes in connection and causes the confusion in router so it is turned off by default but you can use it if you want to.

Load is similar to reliability problem, it detects for the business of connection that how busy a line is? It also uses continuous resource and the decision could be false, because the loads in a connection continuously changes. For example if you are not using network then the load on line is 0 but as soon as you visit a webpage the loads gets to its peak for a second. This parameter is also turned off by default.

MTU is the only parameter that has been eliminated from metrics lists and you cannot use this anymore.



This is the formula behind the metric. Here K values are numbers either 1 or 0. The default functionality of this formula uses K1 = 1 and K3 = 1 and all other Ks are set to 0. The confusion in this formula could be that if K5 is 0 by default then the entire equation gets value of 0 and formula could stop working, so the Cisco has made an exception for K5 that if it is 0 then the reliability equation gets ignored.

The following figure is showing the default values set for Ks.


The formula gets simplified into the following equation.


The delay is measured in Microseconds and bandwidth is measured in Kilobits. The result of this metric is a large number and it cannot be reverse-engineered, meaning that you cannot find the delay or bandwidth from result.

Route Summarization

It is a great technique that summarizes the routing table entries and makes routing more efficient. In EIGRP this technique is by default turned on and it performs the summarization. Summarization performs super-netting of class-full networks and makes the routing easy as shown in the following figure:


You can always turn off this feature by executing no auto-summary.


Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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