3Com S7906E Configuration Manual page 1971

Analysis
Changing port security mode is not allowed when an 802.1X-authenticated or MAC authenticated user
is online.
Solution
Use the cut command to forcibly disconnect the user from the port before changing the port security
mode.
[Switch-GigabitEthernet2/0/1] quit
[Switch] cut connection interface gigabitethernet 2/0/1
[Switch] interface gigabitethernet 2/0/1
[Switch-GigabitEthernet2/0/1] undo port-security port-mode
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