Mobile ad hoc networks (MANETs) raise new challenges in designing protocols for solving the consensus problem. Among the others, how to design message efficient protocols so as to save resource consumption, has been the focus of research. In this paper, we present the design of such an efficient consensus protocol. We consider the system model for MANETs with host crashes, but equipped with Chandra- Toueg?s unreliable failure detectors of class ..P. At most f hosts can crash where f \le n/2 (n is the total number of the hosts). The protocol adopts the coordinator rotation paradigm to achieve consensus. Unlike existing consensus protocols, the proposed protocol is based on a two-layer hierarchy with hosts associated with proxies. At least f+1 hosts act as proxies and each host is associated with one proxy host. The messages from and/or to the local hosts of the same proxy are merged so as to reduce the message cost. Moreover, the hierarchical approach can improve the scalability of the consensus protocol. Performance analysis shows that the proposed protocol can significantly save cost compared existing protocols.