IEEE 802.15.4-2006 represents a widely used standard for multihop wireless sensor networks. However, the standard exploits a tree structure in the MAC layer, which may lead to network partitions even after a single link or node failure, i.e. the well known single point of failure problem. Besides, the single path approach avoids the routing protocol to select by itself a next hop based on its own criteria. Moreover, transmissions must be appropriately scheduled in the IEEE 802.15.4 cluster-tree to avoid collisions. In this paper, we propose to modify the cluster-tree structure into a Cluster-Directed Acyclic Graph (DAG) to improve the robustness and the topology redundancy at the MAC layer. We also present a simple greedy scheduling algorithm integrated with the IEEE 802.15.4 MAC mechanisms. Simulations show that the proposed mechanisms optimize the MAC layer for multihop topologies. In particular, the routing protocol (e.g. RPL) is able to exploit efficiently the cluster-DAG and to reduce the number of packet losses and the end-to-end delay. Last but not least, the cluster-DAG structure leads globally to energy savings by reducing the number of transmissions at the MAC layer.