Those of you not familiar with GNS3, it’s a graphical front end for the Dynamips router/IOS simulator. It’s open source, temperamental, and a resource hog. And, up until two weeks ago I figured I was the only person on the planet that could not get it to work.
I started experimenting with GNS3 last summer. Since then, I’ve tried running it on three different PC’s, encountering the same problem each time. It would run for about 20 minutes, just enough time to get a configuration built and the idle PC values worked out, then it would lock up.
I had all but given up on GNS3. But two Saturdays ago I decided to give it another try.
This time, I installed GNS3 0.6.1 in an Xp virtual machine running in Parallels on my Macbook Pro. I gave the VM 1.2GB of RAM and uninstalled all of the unnecessary cr@p from Windows to keep the install as clean as possible.
Well….GNS3 ran flawlessly!!. I actually spent about five hours with it that afternoon modeling a customer BGP configuration. (five routers)
At first I was still having a number of issues with the idle PC values. I’d keep calculating a new value and applying it, but after a few minutes the CPU utilization would go to 100% again, sending the fans on the Macbook Pro into overdrive!
It wasn’t until I started a console session into all of the routers and tiled them on the screen that I stumbled on what was happening.
In IOS, the default exec-timeout on the console connection is 10 minutes, after which you are returned to a “Press RETURN to get started” prompt. It seems GNS3 or Dynamips doesn’t like this. As soon as a console session on one my routers would time out, the CPU would spike to 100%.
Exiting out of the console session didn’t help, only hitting the return key or powering off the router would return the CPU utilization to normal.
Applying exec-timeout 0 0 to line con 0 solved that issue. Since I’ve started applying that setting to all of my GNS3 router configs, everything runs great.
In closing, I’m excited about GNS3 again. Two of my coworkers are also running it in a virtual machine with similar success.
