Never I have seen this issue but there could be possibilities that someone might have logged into the switch and change the mode to non-tranparent.
Kindly check the logs to see if there is any message as mentioned below:
%SVCLC-5-FWVTPMODE : VTP mode is set to non-transparent
Explanation The VTP mode is not transparent. VTP may dynamically modify VLANs,
including those that are tied to the firewall. That means someone changed the mode of the
switch at that time to non transparent. So thats why you lost some of the vlans.Its not
possible that switch can automatically changed the mode,someone has manually changed the
mode to non transparent and in that state VTP modified the VLANs dynamically.
This is what maximum I can say with the information that you provided me,because we have
lack of complete logs.
Within a stack as in using the switch stack cables on the back? If so, vtp wouldn't matter as those are treated as one switch. If you have other cisco switches in your network I would check the vtp on those. I'd also log any vtp messages and review.
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