Description: The index that identifies the the sub tree in the
vlanPortGroupTable that should be walked to retrieve
the group of ports in this VLAN.

IMPORTANT: This table has slightly different semantics than
the next three objects because in all 802 VLANs, in the end
they all resolve down to a group of ports.

Therefore, this object has the following semantic based on
its value and the value of vlanType:

   vlanType    vlanPortGroupInstance    mode
   ------------------  -------------------------------  ----
   Port-based set      points to a user configured      Manual
                       set of ports
   Port-based not set  0 - There are no ports assigned  Auto
                       to this VLAN at this time.
   Port-based not set  non zero - points to a system    Auto
                       (agent) maintained list of ports