LevelOne IGP-1271 - Page 208

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 Forbidden VLANs :
A port may be configured to never be member of one or more VLANs. This is particularly
useful when dynamic VLAN protocols like MVRP and GVRP must be prevented from
dynamically adding ports to VLANs.
The trick is to mark such VLANs as forbidden on the port in question. The syntax is identical
to the syntax used in the Existing VLANs field.
By default, the field is left blank, which means that the port may become a member of all
possible VLANs.