GNS3 Quick Overview – Step by Step Guide to IOS and IOU

GNS3 stands for Graphical Network Simulator-3. It is an open-source Network Simulator which is mainly used for CCNA, CCNP, and CCIE exam preparation. GNS3 Network Simulator first released in 2008 and at that time it had support for Dynamips.  At that time, the main purpose of GNS3 had to run only Cisco IOS (Cisco Routers).

But, in 2012, GNS3 changes the whole scenario. GNS3 introduced many new features. Now, it is a cross-platform Network Simulator. Now, it is easy to install in different Operating systems like Windows, Linux (Ubuntu, Mint, Fedora) and MAC. Along with this, it introduced IOU (IOS on Unix), which gives additional support of IOU devices like switches.

Now, with the help of virtualization (VMWare / Virtual box), we can configure it as our real labs. We can install many real servers on VMWare and call them in GNS3 whenever we need. Thus, we can practice in a more better way for any exam like CCNA, CCNP or CCIE. Also, with the help of Virtualisation, we can configure Firewalls (Network Security Appliance’s) and use them in GNS3. Introduction of virtualization makes it much better. According to my own views, we can configure many firewalls (i.e. ASA, pfSenes, Palo Alto, Checkpoint) in VMWare, and use them in our projects which are created in GNS3.

How to use  Routers in GNS3

As we know GNS3 is an open-source Network Simulator which support Dynamips (Dynamips is an emulator computer program that was written to emulate Cisco routers). So, with the help of Dynamips, we can import any router image which is with .bin extension.

Steps to Importing an IOS Router Image :

  1. First of all open GNS3 Network Simulator
  2. Go to Edit >> Preference
  3. Go to IOS Router under Dynamips
  4. Click New and then select New Image
  5. Browse for an IOS Router Image
  6. Now, just give the name and import various Interface according to your needs.

Creating your first Network topology in GNS3

So far, we came to know what is GNS3 where we use GNS3. In this section, we will create our first Network Topology, with the help of Cisco 7200 Series Routers. For this, first, open GNS3 and create your First Project.

GNS3 New Project
GNS3 New Project

After, creating a new project. Now you just have to select your Network appliances present in the left-hand pane. In this lab, I’m just taking four Routers of the Cisco 7200 series. Now, by double-clicking, you got the CLI (Command Line Interface) of the router on your Terminal or Putty(A application to take CLI of various devices).  After that, you can configure their interface and various protocols. (i.e. Routing protocol or DHCP, DNS, etc.).

First Network Topology

Getting Starting with Command Line Interface (CLI) in GNS3

In GNS3, after adding your first IOS (Internetwork Operating System) of the Router, you need to create a Topology as given in the above picture. Now, you need to start the appliance either by right-clicking on Network Device or clicking on start button given below the Main Menu. After the starting of IOS, you just need to double click in order to get CLI (Command Line Interface) of that Device.

CLI of a Router in GNS3

Basic Commands which can be used in GSN3

After getting the CLI of a Cisco router, you have to use commands on CLI so that you can configure your desired network topology and can work smoothly with this. Some basic commands for a Cisco Router are given below :

1. Changing the Hostname of a Router :
In GNS3, by default, the hostname of a Router is either R(1) or Router(1). When you take more than one router, the number (1) is increased by the number of routers you takes. Below command can be used to change the hostname of a Router using CLI :

R1#configure terminal
R1(config)#hostname GNS3Network

2. To know Router Interfaces :
If you want to know a total number of interfaces in a router, just type below command and you will get information about all interfaces.
show IP interface brief

2. Assigning an IP address of an Interface :
We must have to assign an IP address in order to get communicate with other network devices like Router, Switches and End devices. You can assign IP address in configuration mode. You can follow below command to assign an IP address of an Interface:
GNS3Network#configure terminal
GNS3Network(config)#interface FastEthernet 0/0
GNS3Network(config-if)#ip address
GNS3Network(config-if)#no shutdown

How to use  Cisco IOU Images in GNS3

Many times, our requirement is to create a network topology with a layer 2 or a layer 3 switch.  By default, In GNS3 have a sort of Unmanageable switches. We can’t configure them with the help of CLI. Therefore, we need Cisco IOU (IOS on Unix) to add Layer 2 or Layer 3 switches. Below steps will guide you to configure IOU Switch in GNS3 :

Importing a Cisco IOU Image in GNS3

First of all, you need a Cisco IOU Switch image with Licence given by Cisco. To add IOU switch, first, you need to import IOU Switch image to GNS3. This can be down in the following steps :

  • Open GNS3 Network Simulator
  • Go to Edit >> Preference
  • Click IOS on UNIX and Enter valid License

    Cisco IOU License Details
  • Go to IOU Devices under IOS on UNIX
  • Click on New and browse for IOU Image

    How to add cisco IOU in GNS3
  • Now Just give a Name to this Switch and click on finish.

Creating the First Project with Cisco IOU

Now, times to create our first project with the help of Cisco IOU. For, this, just create a simple project. Now, take a Cisco IOU, which is just imported, and make a topology with the help of that IOU Switch. Here, I’m just taking two switches and creating VLAN (Virtual LAN’s) on that.

IOU Switches in GNS3.

Creating VLAN’s on Cisco IOU Switch

After the design of Network topology with the help of Cisco IOU, now we try to create VLAN’s so that we can verify the working of these switches. We can configure VLAN’s same as we configure in Real Network Switches. Below command will be used to configure VLAN’s

L2-Switch2#configure terminal
L2-Switch2(config)#hostname GNS3Network2
GNS3Network2(config)#vlan 10
GNS3Network2(config-vlan)#name GNS3_Network
% Applying VLAN changes may take a few minutes. Please wait…
GNS3 Cisco IOU Switch console in RoxTerm


In this article, we read about GNS3 (Graphical Network Simulator). Here, we first cover various versions of GNS3 and their specifications. We come to know, that how we can import Cisco IOU and IOS. Also, we came to know about beginner commands that can be run in Cisco IOS.


If you have any issue in configuring or installing IOS or IOU in GNS3, comment in comment box.

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