I have fortigate 300C, since today morning I am getting error NAT port is exhausted and also I observed high bandwidth consumption due to which links were getting flapped. I have search on google but not found any satisfactory answer.

Can anyone help me to understand why this is happened, and what should I do to troubleshoot.

1 Answer 1


This message means that all high ports (>1023) are used up for outbound NATted connections. This is very rare and signifies that your FGT is processing way too many sessions. Check the number of open sessions in the GUI or CLI (get sys perf stat), "average sessions" and "average sessions setup rate".

Two main reasons may apply: either you've hit a firmware bug or some host(s) on your internal network start sessions wildly, possibly malware induced. High (outbound) bandwidth consumptions hints into the malware direction, too.

Remedy: reboot your FGT. Watch the session count and buildup rate. Depending on the firmware version used, check FortiView to determine where the majority of sessions either originate or are destined to. Then follow the trails.

You may find more hints and help on the Fortinet forum (forum.fortinet.com).

  • Thax, This issue has been resolved. One of our PC is start sessions wildly due to which High (outbound) bandwidth consumption hints. I disconnected that system from network and now everything seems to be normal. Jan 25, 2017 at 14:13
  • @NishadMorey, if the situation is resolved and this answer is useful, then you should accept it, or you should post your own answer and accept it, otherwise the question will keep popping up forever, looking for an answer.
    – Ron Maupin
    Jan 25, 2017 at 15:17

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