Quick GNS3 Configuration Guide


Download and install GNS3 – it has a lot of dependencies but the installer contains of all of them. Accept all the defaults for the programs it installs (essentially just keep clicking next).

Create Project and Image directories using something simple, like G:\Cisco\Projects and G:\Cisco\IOS. Copy the IOS images (ending in .BIN) to the IOS folder.

Open GNS3, cancel the screen asking to open a project and then click Edit then Preferences. Fill in the project and image directory settings by browsing to the folders you just created:

Click OK to save and exit.






Click Edit then IOS Image and Hypervisors

Click the “…” button next to Image file and browse to the IOS folder and choose the image you want to add (the c7200 for example). Double-click the file and then click Save to add it to the list of images.



Click close and test the configuration by dragging a Router c7200 from the Nodes Types menu to the main screen. Click OK to ignore the baseconfig.txt warning message.



Right click on the router you’ve just added (R1 in the screenshot above) and click Configure. Select the Slots tab and add, for example, a PA-2FE-TX (2 Fast Ethernet ports) to slot 1 and a PA-8T (8 Serial Ports) to slot 2:

Click OK.


Now right click on the router and select Start. Right click on the router and choose Console to open a PuTTY session on the console port:



Wait until it prompts for the ”System Configuration Dialog” and then go back to GNS3’s main screen, right click on the router and then choose Idle PC. Wait while it calculates a value and then choose one from the dropdown list with an asterisk (*) next to it. If none of the entries have asterisks, run Idle PC again (it may take a couple attempts for it to find a good value). 

NB: This is an important step, failing to set an appropriate Idle PC value means the emulator will wind up using 100% of your available CPU time. The value usually only needs to be calculated once for each type of router you use.
Once an Idle PC value has been applied, you can add a second router by dragging another c7200 from the Nodes Types menu. Configure its slots as per the first router and then right click it and choose Start.

To link them via serial cable, click the icon on the menu bar and choose Serial from the dropdown list. To connect the cable, click on first R1 and then R2. Hovering over the router icon with your mouse will give you a list of which ports are connected – Serial1/0 in the screenshot below:

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