A method by a wireless network device in a wireless data network comprises: joining a non-storing mode destination-oriented directed acyclic graph (DODAG) in response to receiving a multicast DODAG information object (DIO) message originated by a root device; generating and transmitting a unicast destination advertisement (DAO) message destined for the root device and indicating the wireless network device has joined the DODAG; advertising as a subroot of a subDAG in the DODAG, based on outputting a second message specifying subDAG information identifying the subDAG; receiving a second unicast DAO message generated by a child network device in the subDAG and addressed to the wireless network device, the second unicast DAO message indicating the child network device has joined the subDAG; and generating and sending a third unicast DAO message to the root device specifying the child network device is reachable via the wireless network device.