The IETF ROLL WG is currently in the final steps of the specification of RPL, a new routing protocol for low power and lossy networks (e.g. wireless sensor networks). RPL may use layer two- and layer three-based mechanisms for neighbor reachability maintenance. Since layer two mechanisms may not always be available, RPL relies by default on the 6LoWPAN Neighbor Discovery, a version of the IPv6 Neighbor Discovery which is optimized for LLNs. This paper provides an analysis of the impact of various RPL and 6LoWPAN Neighbor Discovery parameter settings on the link availability and end-to-end path availability, and the related message overhead. Results show that careful tuning of the relevant parameters is critical for obtaining good network performance.