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A microgrid central controller (MGCC) is a conventional approach to achieve power management for microgrids. It is inherited from the matured power system control structure where the generations are concentrated. However with the nature of distributed energy resources (DERs) in microgrids, a centralized controller may not be advantageous anymore. In addition to the computational burden and heavy communication traffic, any communication breakdown or malicious attack on MGCC could pose a high risk of breaking down the entire microgrid. A distributed optimal power allocation algorithm is proposed in this paper to decompose the MGCC functionality and distribute it among each of the DERs. Each DER controller is able to work cooperatively to achieve the optimal power allocation through only limited information exchange with its neighboring DER controllers. A new distributed bisection λ-iteration algorithm is proposed to achieve an optimal power sharing among all the dispatchable DERs. Simulation results with an MG test system consisting of several types of DERs also demonstrate and verify the effectiveness of the proposed distributed optimization strategy.