Advanced Metering Infrastructure (AMI) has currently become the most popular element in smart grid implementations both in home area network (HAN) and Neighborhood Area Network (NAN) environment as well as in large commercial/industrial establishments. The security of AMI has been an issue for several years, and many tools and utilities have been proposed to ensure the security of AMI networks. However, no network is completely safe from malicious users (hackers). Smart Meters (SM) in the NAN are typical targets for attackers, their objective being the acquisition of authentication information and attempting to successfully authenticate to become a part of the NAN. Such attacks are easy to launch and can cause significant impact since false data can be injected into the system. We explore the impact of such an attack on a previously developed authentication scheme and demonstrate that packet replays at a very fast rate can drain resources in a fashion similar to a Denial of Service (DoS) attack. The effect is pronounced since the authentication scheme uses a multi-hop path to reach the central authentication server. The intermediate nodes partially process each packet before forwarding it, causing an increase in the end-to-end delays as well as increased energy consumption. The authentication scheme is coded in C and the replay attacks are launched using an existing open source security tools.