The unique identifier assigned by the sub-group originator
for this sub-group of this P2MP tunnel as received on a Path
message by a transit or egress LSR.
The value of the sub-group identifier used on outgoing Path
messages is found in cmplsTeP2mpTunnelDestSubGroupID and is
copied from this object unless this LSR is responsible for
changing the sub-group ID.
At an ingress LSR there is no received Path message, and
this object should return zero.