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234 Rockwell Automation Publication 1783-UM007I-EN-P - December 2017
Chapter 3 Configure Switch Features
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A Cisco pre-standard powered device does not provide its power
requirement when the switch detects it. A port that is not configured for
PoE+ allocates 15.4 W as the initial allocation for power budgeting. A
port that is configured for a PoE+ switch allocates 30 W.
The initial power allocation is the maximum amount of power that a
powered device requires. The switch initially allocates this amount of
power when it detects and powers the powered device. As the powered
device negotiates power levels with the module through CDP
power-negotiation messages, the initial power allocation can be
adjusted.
The switch monitors and tracks requests for power and grants power only
when it is available. The switch tracks its power budget, which is the amount of
power available on each PoE port. The switch performs power-accounting
calculations when a port is granted or denied power to keep the power budget
up to date.
After power is applied to a PoE port, the switch uses CDP (if CDP is
supported by the powered Cisco end device) to determine the actual power
consumption requirement of the connected powered devices. The switch
adjusts the power budget accordingly. The switch processes a request and either
grants or denies power. If the request is granted, the switch updates the power
budget. If the request is denied, the switch verifies that power to the port is
turned off, generates a syslog message, and updates the status indicators.
Powered devices can also negotiate with the module for more power.
If the switch detects a fault that is caused by an undervoltage, overvoltage,
overtemperature, oscillator-fault, or short-circuit condition, it does the
following:
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Turns off power to the port
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Generates a syslog message
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Updates the power budget and status indicators
Power Management Modes
PoE ports support these modes:
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Auto (default)—The port automatically detects if the connected device
requires power. If the port discovers a connected powered device and the
module has enough power, the port does the following:
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Grants power
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Updates the power budget
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Turns on power to the port on a first-come, first-served basis
–
Updates the status indicators