GNS3 build a cisco lab for FREE!

In my last article I briefly introduced you to Dynamips and GNS3 and explained you how to install and run for the first time this wonderful software.

In this article I'll describe in deeper detail the configuration of GNS3 and will explain you how to build a fully working topology for your CCNA/CCNP lab. Before we start let me list the different model of Cisco routers supported in Dynamips, this is to help you to download the right version of IOS :

1710 1720 1721 1750 1751 1760 2610 2610XM 2611 2611XM 2620 2620XM 2621 2621XM 2650XM 2651XM 2691 3620 3640 3660 3725 3745 7200

Ok for the purpose of this post I'm going to use a 7200 image but keep in mind you can use any image that you like (at home I'm running lab with 2611XM images, the same I have on my "real" routers). For our example we're going to build the following topology :

Looks great eh? Ok let's get to work and start building the topology!

Ok before we begin to build our topology we need to calculate the "idlepc" value for the particular IOS image we're going to use, this is very very important, we need to let GNS3 calculate IDLE PC values for our routers, as you have noticed when you started the routers your CPU usage jumped to 100% near that (the picture is from one of my notebooks an HP Pavillon with AMD64 Turion processor and 2 GB of memory) :

This is because Dynamips, the core emulator running under GNS3, does not know when the virtual router is idle, and when it is performing useful work. The “idlepc” command performs analysis on a running image to determine the most likely points in the code that represent an idle loop in the IOS process. Once applied, Dynamips “sleeps” the virtual router occasionally when this idle loop is executed. This significantly reduces CPU consumption on the host without reducing the virtual router’s capacity to perform real work. IdlePC values are particular to an IOS image. They will be different for different IOS versions, and even for different feature-sets of the same IOS version. However idlepc values are not particular to your host PC, operating system, or to the version of Dynamips that GNS3 is using. It is possible that Dynamips will not be able to find and IDLPC value for an image, or that the values it does find do not work. If this happens, try repeating the process again.

To determine an IDLEPC value for a given IOS, start GNS3. Drag only one router set up to use the IOS version you are trying to configure. Right-click the router and choose Start. Then right-click the router and choose Console. You may need to press Enter once initially in the Console window. When asked “Would you like to enter the initial configuration dialog?”, enter no and press Enter. Wait for the router to settle down at a Router> prompt. You may need to press Enter once or twice to get there.
After the router has settled down to a Router> prompt, return to the main GNS3 window and right click on one of your routers selecting IDLE PC and let GNS3 calculate the best IDLE PC for your particular configuration :

A few IDLE PC values will be displayed with the ones suiting your needs displaying an * in front of them, try to select the one with a lower value and click on OK. Great! we can continue to configure our topology.

Open up GNS3 and drag our three routers into the topology pane and change their hostname to something meaningful for you (I choose 3 cities I love ;)) by right clicking on the routers and selecting "Change hostname"

Now let's add the needed interfaces to our routers, for the purpose of this topology we need :

- Serial interfaces for the WAN connection

- Fast Ethernet interfaces for future use (optional)

To add interfaces simply right click on one of the routers in the topology and select Configure and then select the TAB Slots as you can see you can  configure one router at time, by selecting its "name" on the left pane, or you can globally configure all the routers by clicking on Routers, from this page select the cards to insert in the virtual slots :

As you can see I've added a FE interface and a PA-4T+, basically a serial interface, to our routers, of course the cards and the number of slots depends on the model/image you're using so you could see something different.

Ok now that we have our interfaces all we have to do is to connect the routers, click on the Connect button and to have a greater degree of control on the configuration select Manual this is not mandatory I use just to be able to choose which interface will be connected to what.

Now click on one of your router and select the interface to which the cable will be connected :

Now, I know you've guessed, click on the other router to select the interface and complete the connection :

Now click on the big red cross, aka the create a link button, to go back to design mode :

If you look at the right pane you'll see how the Topology Summary is now showing the connection we've created :

Ok let's power on our internetwork, it has routers no? so it's an internetwork :) You can simply right click on routers one by one and select Start or click on the Play in the too, time to open a console session to our router and start configuring them (I'll just show you the output of the show interfaces command) :

As you can see all our interfaces are there and working perfectly!

Congratulations! You've just configured your first internetwork with Dynamips now it's up to you to build your topologies and start having lot of fun in the wonderful world of networking!

As usual feel free to contact me for any question or problem about the post or more in general about your studies I'll be glad to answer asap!

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