The unique identifier of the VLAN monitored for
this specific statistics collection.
Tagged packets match the VID for the range between 1 and 4094.
An external RMON probe MAY detect VID=0 on an Inter Switch
Link, in which case the packet belongs to a VLAN determined by
the PVID of the ingress port. The VLAN to which such a packet
belongs can be determined only by a RMON probe internal to the
switch.