Using the GNS3 Network Simulator

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Today we will be taking a closer look at another network simulator that can be used by candidates while preparing for their Cisco exams.

The Packet Tracer, by Cisco, is a really cool tool for CCNA candidates, but it really can't be used by candidates who are preparing for their professional and Expert level certs. The scenarios and labs that are done at these levels are way too complex.

As a professional level candidate you have just 3 options that are available to you.

You can invest the money to build your own lab using real Cisco gear. However, this is the most expensive option that not everyone can afford. Just the cost of the Cisco routers and switches will be enough to make anyone scream, not to mention the added charges to your electricity bill.

Your second option would be to rent rack time at one of the many vendors that provide this service. This is not a bad option. It is way cheaper than building your own lab and you get to practice on real equipment. The only down side to this is having to schedule your lab for an available time.

This might not seem like a big deal to most, but for me, it really is as my mind works differently; I usually have these phases that I like to call my "In the Zone" phases when I'm so focused on my studies that I can't be bothered with whatever else is going on around me. Having to schedule my lab time to a date that is available and not exactly when I would like to do the lab (NOW!!!) can be somewhat inconveniencing. The only available date might be at a time when you just had a bad day, or you're really just not in the mood for studying.

This is where your third option come in really handy. It's called the GNS3 network simulator.

GNS3 Network Simulator

What is GNS3 Really About?

The GNS3 network simulator is free, open source software that can be downloaded and used by anyone. You can access the download at this link.

GNS3 works by using real Cisco IOS images which are emulated using a program called Dynamips.

GNS3 is really like the GUI part of the overall product. With this GUI, users get an easy to use interface that allows them to build complex labs consisting of a variety of supported Cisco routers.

The program that does the real job of emulating the routers using real IOS images is Dynamips. Most people refer to this as the back-end to the whole operation whereas Dynagen is referred to as the front-end. This is mainly because Dynagen communicates with Dynamips using a Hypervisor -- all this put together helps to make the configuration process simpler.

Now, take this added usability and throw in the GUI provided by GNS3, and you've gotten yourself a really powerful, easy to use simulator.


Some Supported GNS3 Features

GNS3 website lists the following as some of the features provided by the simulator:

  • Design of high quality and complex network topologies
  • Emulation of many Cisco router platforms and PIX firewalls
  • Simulation of simple Ethernet, ATM and Frame Relay switches
  • Connection of the simulated network to the real world
  • Packet capture using Wireshark


GNS3 Supported Platforms

These are the current platforms supported by GNS3.

As you can see from the table, you've got quite a list of devices that can be used with GNS3 to build your labs.

This is definitely another great feature of this simulator.

As you all know, with each different model of Cisco devices, you have more or less features supported by that model.

These mostly range from the types of commands supported on the particular IOS you're running for that platform.


Using the GNS3 Network Simulator

Next, we're going to take a look at some screen shots so that you get an idea of what an actual lab looks like.

GNS3 Network Simulator

This is the basic look of GNS3 when you start the program. To the left of the image is where all the devices are listed. From this area you would drag the devices you are going to use for the particular lab to the work area --center of the screen.

GNS3 Network Simulator

Here we have a shot of a Frame Relay lab consisting of 3 routers.

GNS3 Network Simulator

Finally, you can see the results of issuing the show run command.


Some Final Considerations

There is definitely a lot that can be done with the GNS3 network simulator. As you've seen from the screen shots above, you're able to really configure your own labs using the devices you need, thereby giving you exactly the required practice needed for a particular Cisco exam.

However, there are some drawbacks to using GNS3. The main one being that you need your own Cisco IOS images in order to make use of the simulator. GNS3 does not come with built-in IOS images and explicitly states on the front of their page that users must provide their own IOS images.

In order for you to obtain IOS images you will need a CCO account, after which you're able to download the images directly from Cisco's website. We do not encourage getting these images by any other means.

Another drawback would be the amount of CPU resources used by GNS3. Well, to be technically correct, it isn't GNS3 that is actually using up the resources of your CPU. It's actually Dynamips and this can be seen from your Task Manager in Windows.

I've used GNS3 on a number of occasions while preparing for different Cisco exams and when I wanted to test a configuration in the lab before putting it into production. It's really a good tool to have as an aspiring Cisco Engineer and as a network admin in general.

If you would like to learn more about GNS3 and how to configure different labs using it, check out this article that covers both GNS3 and Dynagen configurations or visit the GNS3 website where you'll find access to a number of documentations.

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Jason Harry

Jason Harry is a technology enthusiast and a Systems Engineer with over 3 years of experience in the IT field. He spends most of his time playing video games, reading and researching the latest technological advancements as well as studying for his next IT certification. At the moment he is preparing for his MCTS on MOSS 2007 and MCITP Server Administrator. His long term goals include obtaining CCDA, CCDP and MCITP Enterprise Administrator certifications. Jason is very passionate about becoming one of the best IT Professionals amongst his peers and he dreams of one day being able to be a successful IT consultant. (CCNA, CCNP, CCNA Voice, MCSA, MCP, Net+, A+)