GNS3 – testing configurations without the hardware

I have an upcoming configuration change that involves a fairly complex BGP implementation. I’ll be setting up BGP between two Avaya ERS 8600s, two Cisco Nexus 7010s and two Cisco 3845 routers. A few months back I started looking at setting up a few physical routers in the lab to work through the actual Cisco configuration. That’s when I stumbled upon GNS3, Dynamips, Dynagen and Qemu.

Needless to say I never made it to the lab. I did have a few issues getting GNS3 going but those could probably be attributed to PICNIC (problem in chair not in computer). I also had some difficulties getting GNS3 to save all the router configurations, again probably a shortcoming of not reading all the documentation.


really impressed… while it can’t emulate any of the switches it does a great job of emulating the basic router hardware. The image you use in GNS3 is the actual IOS image that you download from Cisco’s website.

I’ll definitely be adding GNS3 to my list of recommended tools!


Comments on this entry are closed.

  • Dorian August 30, 2011, 4:40 am

    Yep, GNS3 is great, I am always searching a similar tool for avaya equipement…

    • Michael McNamara September 3, 2011, 5:30 pm

      Hi Dorian,

      I don’t think there’s a market for a product that emulates Avaya/Nortel equipment.

      I can understand the limitations of emulating the switch products. I’m not sure if Cisco still spins there own ASICs (hardware) but I believe they do right? I know most of Cisco competitors rely on third party ASCIs from Broadcom and others. Although I believe I’ve heard Juniper is spinning it’s own ASICs for their EX Series switches.

      Thanks for the comment!

  • Flintstone August 31, 2011, 12:43 pm


    GNS3 also allows me to test our BGP and OSPF WAN configurations in a test environment before going live :)

    You can do a little switch emulation by adding a switch module to one of your routers. I also believe Cisco’s packet tracer will also emulate their routers and more importantly their switches?


    • Michael McNamara September 3, 2011, 5:34 pm

      Hi Flintstone,

      I ended up getting burned a little with my upgrade. The Cisco Nexus 7010 switches choked on some of the legacy IOS syntax. I also had to go back and write a prefix-list for the route-maps as opposed to using a simple ACL list. I accidentally stumbled upon GNS3… more so because I didn’t feel like racking all that lab equipment to setup my test. In the end it worked out pretty good.

      Thanks for the comment!

  • Garrick September 2, 2011, 4:29 pm


    Dynamips is a great emulator, best I’ve seen at least. If you can live with a little delay (got it down to 20 ms per emulated router), you can also connect it to actual hardware such as an 8600 or 5520. With Dynamips some of the switch commands are older, such as creating VLANs (uses vlan database). Packet Tracer has more switch command limitations IMHO, but is also good.

    To save your configs, the way I do it is to in the net file point each router’s config line to a text file where you have copied the actual running config. Once you launch Dynamips again, it will load that config file with all of your last saved settings. Similar to where you would write out the config in a text file when loading a new switch/router.

    • Michael McNamara September 3, 2011, 5:44 pm

      Hi Garrick,

      I was going to start playing with QEMU and see if I can get a virtual Linux host up and running on the virtual network, now I just need to find the time.

      I ended up doing an export and import everytime I started and shutdown GNS3.

      Thanks for the comment!

  • GlenP September 3, 2011, 3:30 pm

    There’s also a NXOS simulator code-named titanium floating around the internets, which also has potential.

    Unfortunately it can’t be part of dynamips because the Nexus is not mips-based but I have seen it run in vmware successfully.


    • Michael McNamara September 3, 2011, 5:48 pm

      Hi Glenn,

      Thanks for the information…

      I was able to figure out the NXOS syntax (wasn’t hard at all) and GNS3 helped prove out the design. A NXOS simulator would be interesting.

      Thanks for the comment!

    • ali December 12, 2012, 7:56 am

      hi dear friend
      im looking for NXOS smulator. could you send the download link to me

      • Michael McNamara December 12, 2012, 1:28 pm

        Hi Ali,

        Unfortunately there isn’t a simulator or emulator for Cisco’s NX-OS. You’ll need an actual switch running NX-OS if want to test a design or topology.


  • Forrequi September 5, 2011, 4:50 am

    Always prefer prefix-lists over access-lists when using BGP or using it for route information base (RIB) allows/prunes. Prefix-list can be written in FIB (forward information base) for hardware switching.

  • Bob October 13, 2011, 8:16 pm

    You can simulate most Cisco switch functions with the NM-16ESW for the 7200. There are a couple limitations, like the Etherchannel numbers are limited and you can’t do an RSPAN.

  • May 9, 2012, 3:24 am

    Hi Mike! Am having problem with gns3. I cant save my configuration as startup-config thus have to reconfigure everything each time. How can i overcome thist issue?
    Also my cpu is used 100% even with an idle pc value.Plz help.

    Editor: I’ve removed your email to prevent SPAM

  • Michael McNamara May 9, 2012, 9:35 pm

    Hi Nissy,

    You need to read the documentation carefully… you need to set idle pc value for that virtual router.

    Assuming you’ve done a “write mem” from within the virtual router you need to save the GNS3 project configuration and choose “export” the router configurations.

    Good Luck!