Which command is used to upgrade an IOS on a Cisco router?

The command used to upgrade an IOS on a Cisco router is associated with the process of transferring the new IOS image file from a TFTP server to the router's flash memory. The correct command involves copying the file directly into the router's flash storage, where it can be accessed for the upgrade process.

When you use the command to copy a file from the TFTP server, specifying "flash" indicates that you want to place the new IOS image into the non-volatile storage of the router, which is essential for the router to operate with the new IOS version upon the next reboot. After successfully moving the file to flash, you can then update the router's configuration to boot from the new IOS image.

In contrast, commands that mention "run" or "start" do not correctly reference the process of storing the image in flash memory. Additionally, the configuration command does not relate to the upgrading procedure of the IOS directly and pertains to configuring routing and network settings instead.