HP procurve series 4100gl Management And Configuration Manual page 357

If multiple VLANs are configured, stacking uses only the primary
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VLAN on any switch. In the factory-default configuration, the
DEFAULT_VLAN is the primary VLAN. (See "Stacking Operation with
Multiple VLANs Configured" on page 14-44 and "The Primary VLAN"
on page 11-6.)
Stacking allows intermediate devices that do not support stacking.
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This enables you to include switches that are distant from the
Commander.
Commander Switch
Figure 14-3. Example of a Non-Stacking Device Used in a Stacking Environment
Switch with Stacking
Disabled or Not Available
HP Procurve Stack Management
Operation
Candidate Switch
Member Switch
14-7