In this paper, we present an analytical model of two-tier handoff mechanisms for a hierarchical NEMO system composed of access routers (ARs), mobile routers (MRs), and visiting mobile nodes (MNs). In the proposed two-tier handoff mechanism, service areas of MR and AR are respectively divided into handoff and non-handoff regions. Consequently, concurrent voice/data sessions in a hierarchical AR-MR model can be classified into four types according to the locations of MN and MR within the MR's and the AR's service areas, respectively. We build a mathematical model with 4-D Markov chains to analyze the performance in terms of the system utilization and the blocking probabilities of new sessions and handoff sessions. For the purpose of validation, we conduct experiments through simulation. Simulation results reveal that both the blocking probabilities of new sessions and handoff sessions are very close to the analytical results, particularly when the system utilization is high. The impacts of session residence time and session arrival rates on the blocking probabilities are also investigated.