IDLE PC GNS3 | High CPU Problem Minimize The 100% Usage

What is Idle PC value?

GNS3 Idle PC values are used for controlling and limiting the processor usage of your computer. In backend GNS3 uses Dynamips the software for emulating the real Cisco IOSs. Emulating the real Cisco ISO images causes the high usage and processing of CPU processor to 100%. So here comes the concept of idle PC values, after apply the idle PC value you can minimize the CUP usage.

If you are thinking that why before applying idle PC CPU usage is 100%. For clearing you confusion I am quoting here Jeremy Grossmann. According to Jeremy Grossmann Gns3 Programmer “dynamips attempts to emulate programing instructions one at-a-time as fast as it can, this is resulting up to 100% CPU usage. The Idle-PC is a guess at where the Program Counter might be pointing to an idle-loop in the emulated router. When emulator has visits this PC value Idle-Max times, it suspends itself for Idle Sleepms to allow other processes to get on their tasks.”
Basically Idle PC values find the intervals/time when the IOS is not executing any instructions and puts the router into an idle state. Which results the decreases in CPU usage on your computer. 

How to calculate and apply the idle PC value to Router

There are number of ways in GNS3 1.1 & later version for applying the idle PC values. I am going to discuss these method one by one.
Method 1:
You can set the idle PC value during adding the IOS image in GNS3. During the IOS adding wizard it will asked you for setting the Idle PC value. Remember this feature is only available in new versions. You can add the new IOS for your router from Edit/preference/dynamips/IOS routers.  If you want to learn more about adding IOS in GNS3 you can visit here.

Method 2:
You can change add or change the idle PC value for already added IOS image from Edit/preference/dynamips/IOS routers.  Select your router for which you want to change the idle PC and then click on the edit and go to advance tab and put the new idle PC value there. you can also review the following figure for details.
Method 3:
You can set idle PC value once you have successfully loaded IOS to the router, when you create a topology in GNS3. After starting the all devices in gns3. Wait a little when the router fully loaded.

Right click on the router and select Idle PC, GNS3 start calculating the idle PC value for your router.
It will take some time for calculating the Idle PC, once it is done you will find different values. Idle PC values with * are recommended so apply any idle pc value with *.

After applying the idle PC, if there is not effect on CPU usage then you can retry the same process again. In few cases you have to perform this process for two to three times for getting the ideal value for your router. Once Idle PC value has been successfully applied, you will notice a drop in the CPU usage.

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?
In such situations, stop the GNS3, close it and start again and then perform the steps again for idle PC. You should repeat this process until CPU usage goes down to10-15%. You can also lower the CPU usage by applying the idle PC value without Asterisk symbol if it reduces CPU usage considerably.
I hope this article will help you in lowering the idle PC value of your GNS3. Please share your experience regarding IDLE PC value.

CCNA access list Exam Lab with GNS3 Downloadable Configuration files

CCNA ACL Exam Lab with GNS3:

If you are preparing to attempt the CCNA (200-120) Exam, then you must prepare for access-list, because in every exam there is a lab related to assess-list where you have to complete the configuration in order to make the access-list working. I have created the similar lab in GNS3 so that you can practice it before going into the real Exam.

Lab Question:
A network-Engineer is making the security configuration of the Corp1 router. You have to perform the following task for completing these configurations:

  1. The user on host C must be able to use a web browser to access financial information from the Finance Web Server.
  2. No other hosts from the LAN nor the Core should be able to use a web browser to access this server.
  3. YOU ARE required to apply an access-list with no more than three statements that will allow ONLY host C web access to the Finance Web Server. No other hosts will have web from Finance Web Server.
  4. All other traffic is permitted.

Other Information:
The Core user have the IP-address of
LAN PCs have been assigned with IP addresses range from -
The servers in the Server LAN have been assigned addresses of -
The Finance Web-Server has been assigned with IP address of
Image Courtesy: 

Solution / Required Configurations
You can achieve these required task by following /configuration / commands on router Crop1:
Corp1#configure terminal
Corp1(config)#access-list 100 permit tcp host host eq 80
This command will allow the web traffic to finance web server, Where is the IP address of host C.
Corp1(config)#access-list 100 deny tcp any host eq 80
This command will block the web access of any other host to finance web server
Corp1(config)#access-list 100 permit ip any host
Above command will allow the any other traffic.
Apply the ACL to interface near to destination:
Corp1(config)#interface fa 0/1
Corp1(config-if)#ip access-group 100 out
Corp1#copy running-config startup-config

Verification of Configuration:
You can access the web only from to Finance Web Server.

CCNA Access-list Exam lab In GNS3:

If you have some extra time you can configure this lab in GNS3… J You can also configure it with packet-tracer which is easy as compare to GNs3 But I have configured the same lab in Gns3, you can download these configuration files from here and can complete it with above configurations. Simply download the files (link are given at the end of post), create the topology same like shown in the figure and then upload these files on every router one by one. This will provide the initial practice before going into the real exam.

IN GNS3 For making the communications between Corp1, Lan Hosts (A,B,C,D) and finance servers I have configured the EIGRP AS 100, while in the real lab you have the by default communication between these devices.
For host-C and finance web server and other servers I have used the VMs of VirtualBox. I have used internet NIC with them With following IP configurations:

For creating the web service on finance server I simply install the IIS and hosted a web-page named index.html. You can install the IIS on windows XP from control panel\add & remove feautures\add&remove windows components and the select the IIS and click on next and complete the wizard.
I have created a web-page (index.html) for test and place it on c:\inetpub\wwwroot of finance web-server. After completing the above configuration you can test the configuration with the followiway.
  1. open the internet explorer and access the shared web page with URL “\index.html”, it will be successful from host-c as shown in figure.

  • Now try the same from Host-b page will be inaccessible.

CCNA EIGRP Exam Lab | Addition of new Router in EIGRP Process

Practice EIGRP LAB for CCNA:

Burtaqtech has a small network running with EIGRP as IGP protocol. EIGRP  has the AS number of 12 on all routers. Router MGT is also running static routing to the ISP. Burtaqtech has recently added the ENG router, currently which do ‘not have connectivity to the ISP-router. You have to perform the following tasks for completing the configurations:

Find the faults and correct the router configurations to provide full connectivity between the routers.
Following are the IP-address schemes of all the routers in the following network.

Fa0/0 –
S1/0 –
S0/0 –
S0/1 –

Fa0/0 –
Fa0/1 –
S0/1 –

Fa0/0 –
Fa0/1 –
S0/0 –

Fa0/0 –
Fa1/0 –
Fa0/1 –
Image Courtesy: 

This the very simple Lab to configure in the CCNA Exam, all you have to do to advertise the EIGRP network of new added router “ENG” on MGT router. As MGT is the main router which is connecting all the routers to ISP. In our scenario on ENG the interface which is connected to MGT have the IP address of 192.168.77.x, so we will advertise this network on MGT router in EIGRP 12.

So you are required the following configuration for compleing the above EIGRP lab.
On the MGT Router:
Config t
Router eigrp 12


GNS3 Packet Capture Using wireshark | Configure WIRESHARK in GNS3

How to use the Wireshark with Gns3:

In this article I will show you how to use Wireshark with Gns3 for packet capturing. Wireshark is a packet capturing and network analyzer software also known as sniffer or network sniffer. Wireshark will allow you to see, even under detailed level, what is happening in your network. It is also free, open source, cross-platform. I personally rate it is the best tool to network audit and for packet analyzing and capturing. It has a graphical interface and many options for organizing and filtering information. So, you can see all the traffic passing through a network. Using different filters you can analyze the specific protocol packets, like if you want to analyze the routing protocol handshaking of any protocol you can achieve it with wireshark and Gns3.

Wireshark Features

  • Available for Linux and Windows
  • Live Packet capture from any network interface
  • Displays packets with detailed information
  • You can save captured packets and can analyze it later
  • Import and export packets in different formats
  • Packet filtering information
  • Highlighting packages depending on the filter
  • Create statistics

Wireshark is also helpful for following professionals:
  • For network Administrators who can use it for solving different network problems
  • Engineers can use it for checking different security issues of network
  • Developers can use wireshark for debug and implementation of network protocols
  • Students can use it to learn how different protocol works internally in a network
  • Hackers use it to view equipment configurations as in this case cisco routing capture frames .. J

You can also use wireshark with Gns3 for packet capturing, this is the default packet capturing tool that comes with full GNS3 package. By default it is auto configured with Gns3, you can confirm its configuration from edit/preferences. you can see it default setting in figure, if you have any issue with using wireshark with Gns3 you can match your setting with following setting:
For capturing any interface you can follow these steps:
1.       start you network device in Gns3
2.       right click on the link between the two devices/routers and then click on start capture
1.       it will ask you for interface selection, select the interface you want to capture the packets.

1.       Once you have selected your interface click on OK, this will open the wireshark, where you can see the all network packets movement access the selected interface.

I hope this short article is helpful for you and thank you for reading it.

How to Download & Use Cisco ISO XR in GNS3 with Virtualbox

How to Configure Cisco XR IOS Image in GNS3:

IOS XR is a train of Cisco Systems' widely deployed Inter-networking Operating System (IOS), used on their high-end carrier-grade routers such as the CRS series, 12000 series, and ASR9000 series.
In this article I will show you how to configure or use Cisco XR IOS Image in GNS3. This guide will help you the step by step processor for configuring Cisco XR IOS image in GNS3. So following are the steps:
1.       Download Cisco XR IOS image, I have download it from this link: . I have downloaded the “xrvr-fullk9-4.3.2.vmdk” for this lab.
2.       In second step you need to create a virtual machine from this download file. I have created a virtual Machine using Virtual-Box. Open your Virtual Box and click on new for creating a new VM for Cisco XR image. A wizard with start, in first step you are required to enter the name of Virtual Machine.

3.       In next step you are required to allocate the RAM for your Cisco XR VM. It is recommended to allocate at least 1.55 GB RAM.
4.       In next step you need to select the virtual hard disk for you VM. Choose the options “use an existing virtual hard disk” and browse the cisco XR image that you have download in first step. Click on create this will create a new VM in VM list.

5.       Now open the GNS3 and go to edit/preferences and from virtual machines menu click on new. This will start a new wizard select VM that you have created in Virtual-box and click on finish.
6.       After this you will find Cisco-XR image in GNS3 device list. Drag this image to GNS3 dashboard.
7.       Start the XR IOS, once it is started, right click on device and click on Console.
8.       Cisco XR image will take some time while loading. During loading it will also give you the message “this (d)rp node is not ready or active for login /configuration” you can ignore this message for a while after some time it will ask  you to set username and password. First set the username, in my case I have set it with “root” you can use any other. Then it ask for password type the password it will reconfirm from you.
9.       Once you have set username and password for Cisco XR IOS image in GNS3, it will take back you to login screen. Now login with username and password that you have set in step#8.
10.   Now you are in Privileged mode, type the configure terminal for global configuration mode. Now you can configure your router as per your choice.

How to configure IPV4 on Cisco XR IOS:

Configuring IP address on Cisco XR is a bit different as compare to other IOS. Go to global configuration mode and use the following commands for assigning IPV4 address to cisco XR image. 
config t
interface mgmtEth0/0/cpu0/0
ipv4 address
No shut

I Hope this article will help you in configuring Cisco XR and please also share you experience of using Cisco XR on Gns3. Thank you for reading... J