3Com 4500 Getting Started Manual page 87

If you add a unit to a stack that has previously been manually configured with a
unit ID and this conflicts with an ID already within the stack, then the Switch
with the lowest MAC address assumes the ID in question and the other unit
will automatically renumber.
3Com recommends that you manually assign the unit IDs within the stack if you
wish to have predictability of knowing which units have which IDs at all times.
Stack topology is 'discovered' and the units auto-number their IDs.
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Adding and removing units from the stack does not cause any renumbering to
occur and the stack will continue to work normally. Renumbering only occurs
when the stack is next power cycled if the units are configured to auto-number.
The unit LEDs will display the unit number in the stack, from 1 to 8.
If you are having problems, refer to "Solving Stack Formation Problems" on
page 57.
Unit Numbering within the Stack 87