GNS3 Performance

Every now and then I get questions about GNS3 performance, especially people who run into trouble because their cpu skyrockets to 100%.

There is an easy fix for this, but you just need to know how…(that’s always the case right?)

I made a video for all you people who have trouble with GNS3 performance, on a Quad Core machine with 4GB of memory i’m able to run 12 routers (perhaps even more) without any trouble.

If you want to solve this problem this is what you have to do:

  1. Start GNS3.
  2. Click and drag a router to the topology.
  3. Start your router.
  4. Wait till it’s fully loaded and “idle” (doing nothing).
  5. Click with the right mouse button on your router.
  6. Choose “Idle PC”.
  7. (If you get a pop-up saying your router already has an idlepc value applied, just click yes).
  8. Be patient while it’s calculating an idlepc value…
  9. Now choose a value with an asterisk (*) in front of it and press OK.
  10. Your CPU load should drop drastically by now.
  11. Just to be sure, close GNS3 and restart it.
  12. Every router you start from now on will have a low cpu usage.

Keep in mind you need to do this PER IOS IMAGE you are using!

If you want to see how I did it please watch my video:

If you have any questions please let me know.

Opt In Image
Do you want your CCNA or CCNP Certificate?

The How to Master series helps you to understand complex topics like spanning-tree, VLANs, trunks, OSPF, EIGRP, BGP and more.

Written by René Molenaar - CCIE #41726

You May Also Like

About the Author: Rene Molenaar

René - CCIE #41726 is the creator of where he shares CCNA, CCNP and CCIE R&S labs. He also blogs about networking on


  1. Addition to this great tip: Open up a console port to each router you just started..(and press enter if not double clicking from GNS3) else it will keep eating up 25-50% until you do give it some attention 😉

  2. I follows all these steps but problem is still 🙁 CPU usage is about 80%

    My PC RAM: 2 G
    Processor : 3.07 GHZ i3

    please advise 🙂
    thanks in advance 🙂

Comments are closed.