What is the command to copy a cisco ios to a TFTP server?

You can use the copy flash TFTP command to copy the iOS to the TFTP server as shown next:

Router#copy flash tftp

Source filename []?c2800nm-advsecurityk9-mz.124-12.bin

Address or name of remote host []?1.1.1.2

Destination filename [c2800nm-advsecurityk9-mz.124-12.bin]?[enter]

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!

21710744 bytes copied in 60.724 secs (357532 bytes/sec)

Router#

Just copy the iOS filename from either the show flash or show version command and then paste it when prompted for the source filename.