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On a 7301, I believe it does act as a redundant power supply, and this is often true for many Cisco devices with dual power supplies, but there can be exceptions. For example, on the 6500 chassis, you might use dual power supplies in combined mode, i.e. failure of a power supply would leave the chassis short of necessary power. The 3560X/3750X might also have insufficient power if a power supply fails when running on two (this issue may arise when using PoE).
