This document proposes a method called Elastic Tree to reduce network power consumption in data centers. Elastic Tree dynamically adjusts the set of active network elements like links and switches based on changing traffic loads to satisfy demand with minimum power. It consists of three modules: an optimizer to find the minimum power network subset, a routing module to choose paths, and a power control module to toggle network elements. Different optimization methods are explored, including a formal model, greedy bin-packing, and topology-aware heuristic to reduce network power consumption while meeting traffic needs. When applied to an e-commerce data center, Elastic Tree was able to significantly reduce power consumption in the network.