The Internet of Things is characterized by the large-scale deployment of low power and lossy networks (LLN), interconnecting pervasive objects. The routing protocol for LLN (RPL) protocol has been standardized by IETF to enable a lightweight and robust routing in these constrained networks. A versioning mechanism is incorporated into RPL in order to maintain an optimized topology. However, an attacker can exploit this mechanism to significantly damage the network and reduce its lifetime. After analyzing and comparing existing work, we propose in this paper a monitoring strategy with dedicated algorithms for detecting such attacks and identifying the involved malicious nodes. The performance of this solution is evaluated through extensive experiments, and its scalability is quantified with the support of a monitoring node placement optimization method.