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The power sharing mode has an inherent capability to oversubscribe itself in case of power suppl y failures. That
is, when an imbalance occurs between available power budgets and allocated power, more power is alloca ted
than is available. By definition, the system should never have a negative power budget, but it is poss ible that a
power supply failure brings the power budget below the allocated power. Such a condition will trigger a series of
alarming messages to the console.
Negative budget = Available power < Allocated power
The problem is that a demand for previously allocated (negotiated) power may come at any moment, an d there
may not be enough power available to respond to the previously committed resource.
Cisco StackPower technology has a “strict” mode to keep you from getting into that situation. This mode forces the
power stack to stay in a positive budget; that is, when a power supply failure causes this condition , the power
stack begins shedding power according to preset priority levels until the available power is e qual to the allocated
power.
Redundant Mode Redundant mode (Figure 10) reserves an amount of power for redundancy from the power pool and guarantee
operations in case of a single power supply failure in the power stack. The reserved power is equal to th e size of
the largest power supply in the stack. The rest of the power in the power pool is used as in Power-sharing mod e
and the strict and non-strict modes work exactly the same as in power-sharing modes.
Figure 10. Redundant mode
The largest power supply in the stack is not reserved; instead an amount of power equal to the capac ity of the
largest power supply in the stack is set aside as reserved power. In case of a power supply failure, th e entire
power reserve is made available to the budget, ready to be allocated. Note that regardless of which type o r size of
power supply has failed, the reserved power is used as a whole amount and cannot be fragmented.
It is important to note that the Cisco StackPower feature will not take any power supply offline automatic ally based
on the power budget report. That is, a power stack could have a surplus of power capacity and redundancy