! – Indicates a successful transfer to 10 packets O – Indicates out of sequence packets. The characters which show the status of a transfer in IOS are

TFTP is only used significantly today to transfer network device images & associated configurations across trusted networks, it has no methods of listing directories, packet encryption or passwords. There was originally a 512 byte block per packet with a 32MB file limit but this has now increased to a variable block size and a 4GB limit with RFC 2348. TFTP uses UDP port 69, and block numbers within packets to order them. Lets review what TFTP used for and what O means for the file transferring. At that time, upgrading took me more than 30 minutes.

I even have a post “ Upgrade Cisco 3560X IOS Take 30+ Minutes” regarding the process how to upgrade 3560X-48T-S IOS to latest version. I did many times of this kind of tasks before. Switch# copy tftp: flash: Address or name of remote host ? 1.1.1.2 Source filename ? Destination filename ? Accessing t… Loading from 1.1.1.2 (via Vlan1): !!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!! O!OOOOOO!OOOOOO!OOOOOOOO!OOOOOOOO!OOOOOOOOOO… %Error reading t (Timed out) Switch#