We consider the problem of running RPL on top of the IEEE 802.15.4 MAC layer--the two layers operate over two different structures, a directed acyclic graph in the case of RPL and a cluster-tree for ieee 802.15.4. We propose to adapt the cluster-tree of ieee 802.15.4 so that it can efficiently work coupled with RPL. Nodes in our modified cluster-tree can associate with several parent nodes by tak- ing advantage of an adequate organization of superframes at the MAC layer. Building on this modified MAC layer, we define an opportunistic forwarding scheme that extends RPL with the possibility of forwarding packets over multiple paths. Instead of always using a preferred parent, a node opportunistically forwards packets through other parents as long as their routes towards the sink are better. We take ad- vantage of the opportunistic forwarding to support higher- priority delay-sensitive alarms that need to arrive in sink before a given deadline along with low-intensity monitoring data considered as best-effort. We compare our opportunistic version of RPL to its basic version through detailed simulations in terms of packet delivery ratio, incurred delay, and overhead.