GNS3 requires IOS

The IOS of a real device is necessary to emulate the Cisco router with GNS3. Please download the IOS image file of the actual device to the PC that will use GNS3 in advance. The following are recommended models of Cisco routers that are easy to use with GNS3.

  • Cisco3640
  • Cisco3725/3745

While the latest IOS 15.x is not available for these models, you don’t need much of the latest IOS to study for the Cisco certification exam; most technologies in CCNA-CCNP as well as CCIE Routing & Switching can be adequately validated on these routers.

Outside of studying for certification, you don’t really need to use the latest IOS to understand how the basic protocols work.

Advance preparations

Here’s what you’ll need to download the IOS of the real router to your PC

  • Real router
  • PC(TFTP client)
  • LAN cable(UTP cable)
  • Console cable

Your PC will need a wired Ethernet interface. Modern PCs don’t have a serial port, so a console cable that connects directly to the USB port would be useful.

How to download IOS of the real router to your PC

The steps to download IOS of the real router to your PC are as follows

  1. Wiring the real router to the PC
  2. Configure the real router as a TFTP server
  3. Download IOS with your PC TFTP client

Wiring the real router to the PC

Wire the actual router and PC to which you want to download IOS. Connect the Ethernet interface of the router and PC with a UTP cable. Since you need to configure the router, you also need to connect the console.

Fig. Routers and PC wiring
Fig. Routers and PC wiring

When the Ethernet interface is not up, wire it with a cross cable. Or, connect the Ethernet interface of the router and the PC through a switching hub.

The router interface name is FastEthernet0/0 (Fa0/0) as an example. It will vary depending on the actual router.

Configure the real router as a TFTP server

Configure the real router as a TFTP server. First, set the IP address of the interface that is connected to the PC, and any value for the IP address can be used as long as it allows communication with the PC. Then, use the tftp-server command to enable TFTP download of the IOS file on the flash memory.

The following is an example command for FastEthernet0/0 with IP address 192.168.1.111/24 and the file name of the IOS image file on the flash memory is “c1841-ipbase-mz.124-12a.bin”.

interface FastEthernet0/0
 ip address 192.168.1.111 255.255.255.0
 no shutdown
!
tftp-server flash: c1841-ipbase-mz.124-12a.bin
You can see the file name of the IOS image file from privileged EXEC mode with the show version command.
Change the interface name FastEthernet0/0 if necessary.

Download IOS with your PC TFTP client

Download the IOS file from the actual router using the TFTP client on your PC. and you can install an additional TFTP client on Windows 10. Configure an IP address/subnet mask that can communicate with the router and download the IOS file from the command prompt using the tftp command.

To transfer the file “c1841-ipbase-mz.124-12a.bin” with the IP address of the TFTP server is 192.168.1.111, you need to enter the following command.

C:\Users\gene>tftp -i 192.168.1.111 get c1841-ipbase-mz.124-12a.bin
転送を正常に完了しました: 49 秒間に 15788240 バイト、322208 バイト/秒

The downloaded file is stored in the current directory of the command’s prompt.

Related articles