Cisco ASA Series Cli Configuration Manual page 1142

Chapter 1 Configuring Inspection of Basic Internet Protocols
TFTP Inspection
A dynamic secondary channel and a PAT translation, if necessary, are allocated on a reception of a valid
read (RRQ) or write (WRQ) request. This secondary channel is subsequently used by TFTP for file
transfer or error notification.
Only the TFTP server can initiate traffic over the secondary channel, and at most one incomplete
secondary channel can exist between the TFTP client and server. An error notification from the server
closes the secondary channel.
TFTP inspection must be enabled if static PAT is used to redirect TFTP traffic.
Cisco ASA Series CLI Configuration Guide
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