Booting from a TFTP server
Always maintain a copy of the IOS on a TFTP server in case you accidentally or on-purpose erase the flash memory.
When you erase flash, you erase the IOS.
Use the command copy flash tftp to copy the IOS image in flash memory to the TFTP server.
You will be prompted for the IP address of the TFTP server and the source and destination filename.
A typical filename looks like this: igs-j-l.111-5
Type boot system tftp [filename] if you want your system to boot the IOS from a TFTP server instead of the default, which is flash.
You must be in global configuration mode to do this.