The Illusive Art of Acquiring A Good Idle-PC Value

One of the most difficult problems new users have to come to grips with when starting out with GNS3 is the concept of an Idle-PC.

Get it right, and you will have a great GNS3 experience. Try to ignore it, and you will be forever miserable.

Here is my tedious and time consuming method of finding a good idle-pc value.


Step 1:

Windows: Open the windows task manager and sort by %CPU
Linux: Open a console window and enter the command top
Mac OS X: Open a terminal window and enter the command top -o cpu
Keep this window visible for the entire process

Step 2:

In GNS3, start a new topology with 1 router ONLY
Start the router
Open the console. When when the router is fully up, configure the following:

line con 0
exec-timeout 0

NOTE: While writing this post, I observed this step alone dropped the CPU usage from 98% to 1% on a Windows 7 install (running in a VM on OS X)

Step 3:

Back at your task manager or console window:
Take note of the amount of CPU being chewed by dynamips

Step 4:

In GNS3, right-click on the router and choose idle-pc

If NO values appear marked with *, try again

When you find a value marked with a *, WRITE IT DOWN
If MULTIPLE values appear with *, WRITE THEM ALL DOWN (in a column)
… then choose one of them

Step 5:

Check the CPU utilisation for dynamips in the task manager or console window.
Estimate the average CPU consumption for dynamips over say 15-20 seconds
WRITE IT DOWN next to the Idle-pc value you wrote down in step 4

If you have an idle-pc vlaue that shows less than 10-15% CPU, you may want to go to step 6,
Else, go back to step 4

Step 6:

Now that you have a good idle-pc, you need to know how to use it well.

Firstly, check that GNS3 has recorded you best value against the image you are using

That’s in:
Edit->IOS Images and Hypervisors
Select the image you are using and check the IDLE PCs value

Now GNS3 will automatically use that value in any NEW topologies you create.

Step 7:

If you have any saved topologies (ie .net files) that have used this IOS, open the .net file and replace the idle-pc value found there with your new “good” idle-pc value.

Step 8:

Record the results you found (IOS & idle-pc values) in a spreadsheet and keep it!

Now go back to Step 1 and repeat for the next IOS image you use

General Tips for keeping CPU under control

Always use the same image for ALL routers in your topology if possible.
This means using the same router model as well. If this is not possible, use the same image for all routers of the same model.
ALWAYS set the exec-timeout 0 under line con 0
In GNS 0.7.3 and later, you can set a base config for each IOS image under Edit->IOS Images and Hypervisors. Make sure the base config has the exec-timeout 0 under line con 0

About RedNectar Chris Welsh

Professional IT Instructor. All things TCP/IP, Cisco or Data Centre
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