1. GNS3
Graphical Network Simulator

2. Over View
• GNS3 is a Graphical Network Simulator that allows
emulation of complex networks.
• GNS3 allows the same type of emulation using Cisco
Internetwork Operating Systems. It allows you to run a
Cisco IOS in a virtual environment on your computer.
• Dynamips is the core program that allows IOS emulation.
Dynagen runs on top of Dynamips to create a more user
friendly, text-based environment.

3. Get Start with GNS3
Download GNS3.
http://www.gns3.net
The easiest way to install GNS3 in a Windows environment is to use the
top file: GNS3-0.5-win32-all-in-one.exe.

4. Defining Cisco IOS files
You must provide your own Cisco IOS to use with GNS3 due
to licensing issues.
Current platforms supported include:

5. Defining Cisco IOS files - Continuous
On the Edit menu, choose IOS image and hypervisors.

6. Defining Cisco IOS files - Continuous
Under the IOS Images tab, click and then find your
Cisco IOS file and click Open.

7. Using Cisco IOS Decompressed

8. Using Cisco IOS Decompressed- Continue

9. Using Cisco IOS Decompressed- Continue
• Choose the folder which you will extract to.
• Next, find the file you had extracted, then rename this extend
file from .bin to .image

10. GNS3 Main Interface

11. GNS3 Main Menu
Drop-down menu from right-clicking on a router:

12. Calculating Idle PC

13. Calculating Idle PC
Right-click the router and choose Start.

14. Calculating Idle PC
Right-click the router again and choose Console.
A telnet console opens up.

15. Calculating Idle PC
Observe CPU usage in Windows, by pressing Ctrl+ALT+DEL and choose
Task Manager. Click on the Performance tab to view CPU usage.
You will observe CPU usage will
be at or near 50%.

16. Calculating Idle PC
To reduce CPU usage, right-click on
Router and choose Idle PC. GNS3 will
spend a moment calculating an Idle PC
value before presenting the screen to
the right.
If you click the drop-down arrow, you
see a list of possible idlepc values.
Potentially better idlepc values are
marked with an asterisk.
Choose one of the values and click OK.

17. Calculating Idle PC
You may repeat this process to find the value that reduces
CPU usage the most.
The best ideal is CPU usage
around 5 – 7 % per router

18. Building Topology
• Start GNS3. Drag routers running an IOS you have configured into the
workspace.
Right-click each router and
choose Configure.
On Node Configuration
Panel click Slot.
On Slot Drop-Down, choose
the type interface you want.

19. Building Topology
Click on Add Link, and choose Manual

20. Building Topology