Here is a quick and easy way to replace the IOS on your Cisco router, as long as the router supports tftpdnld. Before you do this you will need a IOS BIN file, either from a original working router of the same model etc or from Cisco direct. You can use this procedure if you need to replace your IOS with a new version or if it’s become corrupt. Please note there is different ways to replace and delete a IOS, this is the method I tend to use.
Delete Current IOS
1.Before doing this backup your current config.
2.From the Router# prompt or from ROMMON 1 > locate the name of your current IOS by doing a dir flash: this will show current IOS file name.
3.Delete the IOS file, do del flash:filename.bin and press y to confirm
4.Once you have done this do a dir flash: to list the flash directory you should no longer have a BIN file in this directory.
5.Switch router off, and then back on again, it will now boot to ROMMON 1 >
1.Boot the router in ROMMON mode. To do this power it up, and press CTRL and BREAK at same time, you will then get the ROMMON 1 > prompt. If you have already deleted IOS it will just boot to ROMMON 1 > directly.
2.Ensure that you have your TFTP server running and BIN file in your TFTP directory ready to upload.
3.Once at the ROMMON prompt enter the following commands, line by line with your ip address details
4.Once you enter the last command you will see that your file should start to upload.
5.When the file has loaded switch the router off and back on again and then you will have the new IOS.
Note : If you get an error about insufficient space, delete one of the TAR files from the Flash directory.