This is a unique index for an entry in the
mplsL3VpnIfConfTable. A non-zero index for an
entry indicates the ifIndex for the corresponding
interface entry in the MPLS-VPN-layer in the ifTable.
Note that this table does not necessarily correspond
one-to-one with all entries in the Interface MIB
having an ifType of MPLS-layer; rather, only those
that are enabled for MPLS L3VPN functionality.