Setting Idle-PC value on IOS routers in GNS3

Dynamips is software emulator on which GNS3 relies. It’s main purpose is to emulate real Cisco router hardware appliances on which IOS resides. Although it gives us plenty of options and possibilities of creating various network topologies and environments, there are also some tweaks that are worthy of our attention. In this tutorial, I will show you how to properly set Idle-PC value on Dynamips routers, which is essential for CPU resource allocation on our local machine.

First of all, you should drag and drop one of your Dynamips IOS routers to GNS3 workspace, start it and then open console window (both options with right clicking your router) to verify that router is successfully booted up.



After that, you should right click router again and then choose Idle-PC option. That will present you drop-down list of all possible Idle-PC values that GNS3 has calculated for you.

At this point, you’ll probably notice that one or more calculated values has asterisk sign assigned. That means GNS3 has marked this values as best possible for this IOS image version (If you don’t have any value that has asterisk sign, force GNS3 to calculate Idle-PC values again). You should choose one of them and then click Apply and OK. At this point you have successfully completed Idle-PC value assignment on your Dynamips IOS router.


So, what’s the purpose of Idle-PC value assignment? The point is, Dynamips emulator, on any point in time, doesn’t know if IOS router is in idle state or functional and needs CPU resources from local machine. So, without Idle-PC value, Dynamips is taking almost all of your CPU resources just for that one IOS router. In that circumstances, it’s not only that you won’t be able to create your GNS3 labs with multiple router instances, you probably won’t be able to do anything useful on your machine.

NOTE – You don’t need to calculate Idle-PC value for every router instance in your GNS3. You should only do it once for every IOS image, and that value will be saved for every future instance of that same router.

I hope that this tutorial was informative to you and thank you for visiting this website.

Ivan Sasic

Ivan Sasic

Ivan is IT professional and System Engineer for more then 7 years with experience in financial institutions and education sector. Passionate about computer networks, football and running.

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