The VLAN index of the VLAN which would be the parent for
this VLAN. This object is only instantiated when the value
of the corresponding instance of vtpVlanEditType has a value
of 'fddi' or 'tokenRing' and Source Routing is in use on
this VLAN. The parent VLAN must have a vtpVlanEditType
value of fddiNet(4) or trNet(5), respectively.