I have the same issue. For me it is using the 192.168.1.99 address. Since TFTP is udp I would think it would be fine. It should get there anyway. However, there is another FGT in between my FGT to backup and the TFTP server. The FGTs seem to do a reverse path check. So if the FGT does not have a way back it won't pass the traffic. Not sure if it does that with udp traffic but I have seen it with TCP. Anyway it is funky and backups do not work for me from remote FGTs to my TFTP server. Thinking I will open a ticket.
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