This backup should perform before any IOS image upgrade.In case of unsuccessful IOS image upgrade (due to too-small DRAM size, corrupted IOS image, etc.) it is always a good to know that you are able to reverse the change by putting back the older IOS image (the one that you backup earlier) to the router In order to transfer an IOS image typically you need to run a TFTP program on your computer.Using TFTP server instead of using FTP server is more common however for image transfer.
Later description will show you how to back up the image* There is a chance that a router loses power or loses connection during the IOS image transfer (copy from or to server) which may cause the router to have no working IOS image to boot up from. Try the following* Backup image to TFTP server * Download image from TFTP server For those routers or switches that have USB port, you can use USB Flash drive to backup or to download IOS images as follows* Backup image to USB Flash drive * Download image from USB Flash drive Once you have both old and new IOS images in USB Flash drive, you can either save copies to your computer or to some storage such as DVD, tape backups, or some FTP servers.When such incident happens, you will be glad that you first back up the IOS image to server before deleting working IOS image off the flash memory. To transfer an IOS from PC to ROUTER, the following conditions must meet* IOS image the router to have is in working condition since corrupted IOS image won't do any good* IOS image the router to have meets the router DRAM and flash memory size.Your computer will then become TFTP server during the IOS image transfer.A simple FREE one can be found here»Cisco Forum FAQ »How to prepare TFTP/FTP server Note that when the router is running newer IOS image, IOS image transfer using FTP server is also supported.This FAQ assumes that your router is able to boot up normally.
When your router is unable to boot up normally, please scroll down to the bottom of the page for suggestions.* Under any circumstances, DO NOT delete working IOS image off the flash memory unless the image has been backed up to server (TFTP or FTP server) or you have working IOS image stored in the server.