Select the IDLE PC for the router in GNS3

We will now learn how to find an idlepc value for the IOS we are currently using. This is a very important step. When an IOS is running, it will consume up to 100% of your CPU time. This will cause your computer to become very sluggish and will prevent building more complex topologies. However, if we use an idlepc value, we can reduce CPU usage dramatically. It puts the IOS into a sleep state when it is not in active use and wakes it up only when it is necessary.

Right-click R0 and choose Idle PC. GNS3 will spend a moment calculating an Idle PC value before presenting the screen to the right. If you click the drop-down arrow, you see a list of possible idlepc values. Potentially better idlepc values are marked with an asterisk. Choose one of the values with an asterick and click OK.

How to confirm the IDLE PC value

You’ll receive a confirmation that the idlepc value has been applied.If you choose IOS images and hypervisors on the Edit menu, and double-click on the image under the IOS Images tab, you’ll see the new idlepc value displayed under Settings.

You may repeat this process to find the value that reduces CPU usage the most. To observe CPU usage in Windows, press Ctrl+ALT+DEL and choose Task Manager. Click on the Performance tab to view CPU usage.

You will observe that without an idlepc value, CPU usage will be at or near 100%, but with an idlepc value, CPU usage will drop to a very low value.