Core issue
Network congestion can cause the packets to get lost between the server and the destination device.
Resolution
Try to connect the Trivial File Transfer Protocol (TFTP) server directly to the PIX with a crossover cable.
TFTP is a quick yet unreliable protocol and is therefore subject to problems if packets are lost between the server and the destination device. When you remove these problem areas, it corrects the issue.
Refer to Use the copy tftp flash Command to Upgrade the PIX for more information on how to upgrade PIX with the TFTP server.
