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.

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The OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

There are 7 Layers in the OSI Model, each describing a particular aspect of networking.

OSI Model
Data Link

It is an essential to memorize the layers of the OSI model and what they do.  There are two common pneumonics that are generally used, one starts at the top and the other at the bottom:

All People Seem To Need Data Processing

Please Do Not Throw Sausage Pizza Away

Either one can help you remember, but don’t forget which end you are starting on.  Now that you have them memorized, what does each layer do? Continue reading »

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