Identifies the index of the primary. Manager obtains new index
values for row creation in this table by reading mplsTunnelIndexNext.
When the MPLS signaling protocol is rsvp(2), this value equals to
the value signaled in the Tunnel ID of the SESSION object. When
the MPLS signaling protocol is crldp(3), this value equals to the
value signaled in the LSP ID. Reference to MPLS-TE-STD-MIB.