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What happens with power supply failure depends on model of 3560 or 3750 whether it supports redundant power supplies, whether a RPS is being used, and/or whether 3750-Xs are being used with PowerStack. I.e. there are options to preclude single a power supply dropping taking the 3560/3750 with it. You might also design to avoid one circuit's power loss taking out units too.
For stacked 3750s, loss of single stack member, for any reason, shouldn't take down rest of stack.
BTW, the 2960S and 3850 support stacking too (different technology). (The newer 4500-X also supports two units as a single logical unit using VSS.)
You can also design a redundant core with two standalone devices, e.g. a pair of 3560s, but they will operate as two stand alone devices.
Depending on actual design, a stack or VSS might deal with device failure "better" and might provide additional performance over stand alone units.
