Router Simulation Using GNS3 & Dynamips

I’ve been working in networking for more than 10 years, but doing the job is not the same as proving you know the information.  For me, the first step is taking the CCNA test.  To be completely honest, I am not a fan of tests.  The last time I took the CCNA (7 years ago) I failed by 2 points.  I don’t blame the test, it was totally my fault.  I did not want to study ISDN, and guess what most of the test was…

So, I’m back on the books and practicing the entire body of knowledge – even ISDN (if it’s there).  Even though the labs in the books are *childs play*, it is a good idea to do them at least once.  This helps to ensure you’re thinking at the level of the test.  While I have many routers and switches at work, I can’t exactly implement the lab environment on them.  I also don’t have the money to buy all the hardware myself, a decent lab will run you $300 – $700 on ebay.  So, a simulator is the next best thing – unlimited routers and switches!

I have looked at some of the commercially available simulators, such as Boson & Routersim, both of which are pretty cool.  You get the unlimited routers and switches, but the command sets are not entirely complete.  That’s not to say they aren’t enough for CCNA, but I would prefer the real experience.  Enter GNS3 and Dynamips.

Dynamips is an emulator program that was written by Christophe Fillot to emulate Cisco routers.  The one caveaught with Dynamips is that it uses REAL Cisco IOS images.  This means that, under the Cisco EULA, you must have the right to have the IOS in order to use it.  If you have the right, Dynamips might be the right option for you.  There is another option for using Dynamips, which I will discuss later in the article.  I am fortunate to have one of the routers supported by Dynamips at work.  (I am not certain if I am skirting a grey area here…)

In order to quickly configure your routers, sometimes it helps to have a GUI.  GNS3 seems to be the favorite among many studying for CCNA.  I’m not going to get into the details of installing GNS3, it’s painless and easy to do.  Their website provides easy to follow instructions for installing on Windows and Linux (they seem to favor Ubuntu).

You can get GNS3 here:

Once you have installed, click Read More to see the rest of this article.

So, let’s assume you have access to any one of the supported router modules on GNS3, what can you do?  Let’s start with a simple network and create one router.  You drag the router to your topology and right click on it to configure.  Because it is a “real” router, you have the option to configure any of the same hardware that router supports.

Click OK and add another router with the same configuration.  And now you have two routers that are configured, but not connected.

To connect the routers, click the add link button, select the appropriate type of link, on the toolbar.  Click on R1 and draw a line to R2.  To complete the link, Click on R2.  Once you are done connecting, you need to click the add link button again to get out of the link mode.

For my purposes, I selected FastEthernet.  GNS will automatically link to the lowest numbered available port.  For me that is F0/0 to F0/0.

Now it’s time to power your router on and see what happens.  Right click on the R1 and select Start.  Very quickly right click on it again and click Console.

If you configured your router correctly (remember you have to set the RAM size to at least the minimum Cisco says for the IOS you are running) you should see the router bootup process.

If you do not see the #### part and the router does not appear responsive when you press Enter, you probably need to check your configuration and make sure you have the RAM correct.

If it booted up, go ahead and Start up R2.  Once it starts up, you should have green lights on the devices.

You can right click on each of the routers and console in to configure the interfaces, routing and connectivity exactly as though you were working on a router setup.

Now here comes the cool part.  The guys over at have labs you can download – that are preconfigured for different scenarios.  You save them and load them in your GNS3 and you can practice doing many of the labs that are listed in the CCNA books.

Some thing to think about.  Each router is USING the RAM you specify, so this can really put a load on your computer.  If you create a network that is very large, you can drop your computer to its knees.

Now, suppose you don’t have access to any Cisco IOS images.  Dynamips was originally developed to simulate the Cisco 7200 router – as a result, the creator did create 7200 IOS image – which you can download @

As always, do your research and good luck!


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