1 2 Previous Next 17 Replies Latest reply: Jan 24, 2012 3:47 AM by Erick RSS

    GNS Idle PC


      Hello guys,

      I decided to learn and to master the use of GNS to prepare for the upcoming certifications.

      I have read many guides and video tutorials reported on other posts.


      After mounting the IOS that you see below:




      with these images can not establish a good IDLE PC values because the CPUs are always high and the calculations of 'IDLE PC almost never come with an asterisk.





      Maybe I put the IOS for routers require too much power?

        • 1. Re: GNS Idle PC

          try other ios versions as well... i never liked fiddling with the idle pc values because it took so much time away from actual progress... and the switch support is just not there... for professional certs i'd recommend investing in equipment... that's what i did...

          • 2. Re: GNS Idle PC

            Hi Fabrizio,


            Try to hardcode this value like i have in the picture. I would recomend decompressing the images first. The steps are:


            1- Extract the image to the images folder.

            2- Rename the image name to the IOS name_version with the extention '.image'; like this: for IOS image of c3725-advsecurityk9-mz.124-3.bin, rename it to: c3725-advsecurityk9-mz.124-3.image

            3- In GNS point to that image.

            4- Start a router with that same image and try resetting the idlepc value, or try using the one in the picture. (though if im not mistaken, idle pc values depend on your processor architecture and memory, so my value might not work for you, there is no harm in giving it a try.)


            hope it helps.


            ps: gns will work 95% for route studies not for switching.idlepc.jpg

            • 3. Re: GNS Idle PC

              Uncompressing image may help but is only necessary for 2600s routers (as some of those crash)

              usually there are many values to choose from, select value marked with * or lower value


              here are some good tips https://learningnetwork.cisco.com/thread/23445


              Before you do IDLE-PC, do this:

              - start one router,

              - open one console window,

              - set up exec-timeout 0 0 (or no exec-timeout)

              - save config with wr

              - do IDLE-PC


              Idle-pc is per IOS, and it is saved automatically now.

              • 4. Re: GNS Idle PC
                Greg, CCNP, JNCIP

                When I first got GNS3, I tried it on Windows 7, and it pegged my CPU utilization at 100%.  This was even after setting Idle PC, w/ a simple 4-router network of 3725s, and running on a PC w/ AMD Phenom II x4 (quad-core), 8 GB DRAM.  Then I read somewhere that GNS3 runs better under Linux.


                So now I have GNS3 on Ubuntu Linux, running inside of Virtualbox under Win7, w/ 2 GB DRAM assigned to Ubuntu.  This setup uses more memory than just running GNS3 under Windows, but the weird thing is that it performs much better than GNS3 directly on Windows.  Even running a 10-router OSPF network, not setting Idle PC, my overall CPU usage is between 25%(idle, keepalives) and 80% (establishing neighborships, converging).

                • 5. Re: GNS Idle PC

                  because linux is elegant and windows is a pig...

                  • 6. Re: GNS Idle PC
                    Then I read somewhere that GNS3 runs better under Linux.

                    on weak and old CPU = Yes, but on new ones, no or there is no significant difference between Win and Linux.


                    see my test https://learningnetwork.cisco.com/thread/25577

                    • 7. Re: GNS Idle PC
                      Greg, CCNP, JNCIP

                      In some cases that's true.  In my experience (long and varied), no single solution/vendor is a perfect fit for every need.

                      • 8. Re: GNS Idle PC
                        carlos - CCNP, CCSI, CCNA Security

                        You have to understand that not everybody can afford real gear.

                        • 9. Re: GNS Idle PC

                          understood, carlos, and i truly wish everyone here could...  through the years i spent untold sums of money on books, videos, simulations, etc... ultimately, that money was better served towards building a lab... i think GNS is the most amazing thing that has happened for network learning in the last ten years... i think it incumbent on cisco to partner with that technology, embrace it, and create, not a simulator, not an emulator, but a full blown virtualized environment, and make that available, at a reasonable cost of course, to all who truly want and desire it... 

                          • 10. Re: GNS Idle PC

                            Guys, before you make changes to the IOS, what features do you think is good enough
                            (ipbase - Enterprises - advancesecurity - Telco - etc.etc.) Certifications for the future?

                            for example:


                            1700 -> IOS ipbase , advancesecurity , telco ? ?

                            2600 ->

                            2691 ->

                            3600 ->

                            3700 ->

                            7200 ->


                              Once you have chosen a more or less do the tests with IDLE PC.

                            • 11. Re: GNS Idle PC

                              i dont think you gonna need the 1700. i do not use it and my gns3 can't handle it (at least on my box, i got corei5 x64 and 4 gig of ram).the other ones depending on which certifications are you trying to accomplish, i would recommend "adventerprisek9" for the others but there is a tool which you can research which you should go for:



                              • 12. Re: GNS Idle PC

                                You are not making changes to IOS, you cannot unless you are Cisco guy who have tools like compiler. 

                                idle-pc make changes to GNS3 when you run IOS on your PC;


                                They all support Telephony (not sure about 3600s) with advanced IOS version.

                                For CCNA/CCNP you need version 12.4T advanced with IPv6 and other goodies.

                                • 13. Re: GNS Idle PC

                                  not sure about the 3600 support for Telephony though, never tried it.

                                  • 14. Re: GNS Idle PC



                                    I have found that along with a good Idle PC value, there is another little "trick" youcan use to reduce the CPU utilization.  Here is an excerpt of on of my .net files.  You can add the statement idlemax = 100.


                                    autostart = False


                                        workingdir = working

                                        udp = 10000


                                            image = /Applications/Dynagen/images/c3725-adventerprisek9-mz.124-15.T10.bin

                                            ram = 128

                                            disk0 = 64

                                            idlepc = 0x62b21ea4

                                            idlemax = 100

                                            ghostios = True


                                    I have found that after adding that to my .net files that my CPU went way down when I used it in conjunction with a good idle pc setting.


                                    Best wishes to you in your certification goals.



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