Upgrading Your Router's IOS
TFTP is the easiest way to upgrade your router's IOS. It is recommended that you have the TFTP server on the same local segment, if possible. On RFF routers (1600, 2500 series), the router boots with the flash in read-only mode, and because it reads data structures from the IOS stored in flash, it cannot be altered while the router is using them. To perform an IOS upgrade, you need to configure the router to load its IOS off the TFTP server instead of using the flash stored image.
On RFR routers (1600-R, 2600, 36x0, 4x00, and 7x00 series), the router's flash is in read/write mode all the time because the IOS is loaded into DRAM at boot. IOS can be upgraded while the router is in operation.
There is, however, an ...