Setting up GNS3

Graphic Network Simulator 3 or better known as GNS3 is a open source network simulator that allows you to build and test complex networks. GNS has the ability to emulate many popular routers from Cisco and Juniper

In this section I am going to review the steps to install and configure GNS3, which is very straight forward.


Installing GNS3


To begin with you first need to download GNS3 from I highly recommend that you download the GNS3 all in one. This includes the required Dynamips, Putty, Qemu/Pemu, VPCS, Wireshark and WinPcap.

When you first execute the setup program you will be presented with a list of components to be installed. By default I have selected all of the options available.


GNS3 Setup-01


After continuing on, you will be prompted for the install of each additional program with the first being WinPcap.



The next component to be installed is Wireshark.


GNS3 Setup-03


The setup program pretty much is a step of next, next finish. When all components are installed the GNS setup will be finished and you can now start up GNS3


GNS3 Setup-04

Configuring GNS3

One of the things I like to do before actually running GNS3 is to pin it to the taskbar and set it to run as administrator all the time. This is just to prevent any problems with access to any resources when GNS3 is being run.  To do this, right click on GNS3 and select properties. From the properties page select advanced.


GNS3 Setup-05


From here you will select run as administrator.


GNS3 Setup-06


Now when you run GNS3 you will automatically be prompted for either administrator credentials or authorization.

When you first run GNS3 you will be presented with a setup wizard. Here you can configure the path to where you will store your projects and IOS images.


GNS3 Config-01


I usually store my images and projects on a separate drive, but this is just a personal preference. One thing however you will want to do is test Dynamips. Without Dynamips, GNS3 will not run. Here after a standard install, I can confirm that Dynamips is indeed functional.


GNS3 Config-02



Now when it comes to the actual IOS images, you have to remember that you must own the router of the IOS image you are going to use in GNS3. What that means is that if you are going to use an IOS image from a Cisco 3700 router, you must also own the physical router. IOS is proprietary software! You have been warned!

Usually when you have a IOS image, it will be compressed. That is so it takes less room in flash memory on the router. However you should always uncompres the images first.  GNS3 will uncompress them for you, but you can also use WinRar or 7z to uncompress the images as well.

To add an IOS image to GNS, go to edit menu and select IOS images and hypervisors.


GNS3 Config-03


From here it is just a matter of selecting your IOS image and selecting the default RAM. Be careful when selecting the amount of RAM as I have found that the default amount of RAM is insufficient. In either way, make sure you test the settings to make sure the router will start properly. Also make sure select save before you close the dialog box.

Now if you click on the router icon on the left, you should be able to drag a c3700 router onto the project area.

One thing that must be done with all new routers that you setup is set the idle pc setting. When you start up a new router you may notice the CPU usage is abnormally high. To demonstrate this I will start the 3700 router and monitor the CPU usage.


GNS3 Config-04


Here you can see the CPU being run at 100 per cent with just one router running.  Obviously it would be impossible to create any meaningful network in this scenario. The GNS3 documentation says that with when setting the idle pc, it puts IOS into a sleep state when it is not active and wakes it when the router is needed.

To configure the idle pc value, right click the router and select start if the router is not already running. Right click again and select Idle PC. When this is done, GNS3 will calculate a number of values, the values you want to try are the ones marked with an asterisk ’*’


GNS3 Config-05


I usually select the first value that is asterisked, when this is completed the CPU usage should be brought down to normal values. If the CPU value does not drop down to normal values, regenerate idle PC values and try again.

As you can see installing and configuring GNS3 is pretty straight forward and as long as you follow these steps you can have a really solid workbench to work with in no time.

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