How to Calculate Idle PC Value in GNS3 Network Simulator 0.8.6?

By | May 25, 2014

GNS3 (Graphical Network Simulator) is the best open source Network simulator that can be used while setting up a computer network. GNS3 works by consuming real Cisco IOS images which are emulated using a program called Dynamips. Hence it provides real time experience to the user and serves many of the network engineers today to design and collaborate their networks.

Using GNS3 you can design and experiment multiple networks without having any dedicated network hardware. The key feature of this router simulator is that you will be able to connect real networks to virtual GNS3 labs! GNS3 simulator is available for Windows, MAC, and Linux platforms.

How GNS3 serves you?

  • Design of high quality and composite network scenarios.
  • Supports many Cisco routers and PIX firewalls.
  • Simulation of simple Ethernet, ATM and Frame Relay switches.
  • Communicating simulated network to the real existing networks.
  • Packet capture using ‘Wireshark’

In this tutorial I would like to explain How to Calculate an Idle PC in latest version of GNS3 0.8.6 running on Windows 7? The process of setting up anIdle PC value is the extreme basic step while working with GNS3. The concept of an Idle-PC is one of the basic problems that a beginner faces while getting started with GNS3.

What exactly is Idle PC value in GNS3?

While simulating your network in GNS3, there is a possibility for 100% CPU usage of your computer. This is due to the Dynamips, the core emulator, doesn’t know when the virtual router is idle, or when it’s executing tasks.

Idle PC value calculates the time when the IOS image is idle (not performing any task) and puts the router into an idle state (sleep mode).

Hence it significantly decreases CPU usage on the host without losing the virtual router’s capacity to do real work.

Step By Step Procedure to Configure Idle PC Value

Well, let’s get into the procedure to be followed for calculating a good idle PC value in 0.8.6. After installing GNS3 load your IOS image to GNS3 virtual router (this process is out of the scope of this article)

Step 1:
Once you have successfully loaded IOS to the router, create a topology with a single router. Then start the router by clicking ‘Start/Resume all devices’ button.
GNS3 idle pc
Step 2:
Wait a little bit for the router to settle down. Double click on the router and verify that it has completed initialization.
set GNS3 idle PC

Step 3:
Openthe task manager (Shift + Control + Escape) and look at the CPU usage,there’s a probability that it may reach 100%.
GNS3 idle pc value
Step 4:
Go to the main window of GNS3, Right click on the router and select Idle PC
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GNS3 will now calculate better Idle PC Value for your host (It may take a title bit time).

Step 4:
After the calculation GNS3 gets prompted with available Idle PC values.
Probably better idle pc values are marked with an asterisk symbol (*). Choose one of the values with an asterisk.
GNS3 5
You’ll get a confirmation that idle pc value has been applied.
GNS3 6
Now Idle PC value has been successfully applied!
Once Idle PC value has been applied you may notice a striking drop in the CPU usage.
GNS3 7
Navigate to Edit → IOS images and hypervisors or hit Ctrl+Shift+I together
GNS3 8
You will find the calculated Idle PC value there under Settings.
GNS3 91
Same value will be applied automatically next time when you add the same router.

Common GNS3 Idle PC Problem: “Could Not Find Asterisk in the Idle PC”

This happens commonly, what do we do if there is no asterisk symbol near to idle PC value?
Insuch situations,stop the simulation,start again and repeat the above procedures. You should repeat this process until CPU usage goes down to10-15%.

NB:-  You may also select values without Asterisk symbol if it reduces CPU usage considerably.

Now you can start creating new topologies and explore with GNS3. Hope you guys got the complete idea about GNS3 Idle PC setup and get rid of the100% CPU usage.

2 thoughts on “How to Calculate Idle PC Value in GNS3 Network Simulator 0.8.6?

  1. fer

    Good stuff. To the point and best of all, worked for me. Cheers.

  2. sambhav

    How can I apply IDEL PC value when we use different IOS in same topology, for example R1 is 3725 series and R2 is 7200 series. then how can I reduce processor utilization.


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