Primary index into mplsLabelStackTable identifying a
stack of labels to be pushed beneath the top label.
Note that the top label identified by the out-
segment ensures that all the components of a
multipoint-to-point connection have the same
outgoing label. A value of 0 indicates that no
labels are to be stacked beneath the top label. This
object cannot be modified if mplsXCRowStatus is
active(1).