Typically, a router has one IOS image and that is the IOS that is used. This IOS image is typically stored in
Flash memory, which is a rewriteable, permanent form of storage. The IOS image can also be placed on an
external TFTP server, but this is typically done for testing. In the IOS upgrade process you first must obtain
the IOS image from Cisco. Then you must place the IOS image into the default directory of a TFTP server.
Finally, you must use the copy tftp flash command from the router to copy the files into Flash memory.
During this process, the router will need to discover the IP address or host name of the TFTP server; the
name of the file; the space available in Flash memory for this file; and whether you want to erase the old
files. The router will prompt you for answers, as necessary. Afterward, the router erases Flash memory as
needed, copies the file, and then verifies that the checksum for the file shows that no errors occurred in
transmission. The show flash command then can be used to verify the contents of Flash memory. Before
the new IOS is used, however, the router must be reloaded.