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11-8
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter 11 Configuring Interface Characteristics
Understanding Interface Types
the request is granted, the switch updates the power budget. If the request is denied, the switch ensures
that power to the port is turned off, generates a syslog message, and updates the LEDs. Powered devices
can also negotiate with the switch for more power.
Note The CDP-specific power consumption requirement is referred to as the actual power consumption
requirement in the Catalyst 3750 and 3560 software configuration guides and command references.
If the switch detects a fault caused by an undervoltage, overvoltage, overtemperature, oscillator-fault,
or short-circuit condition, it turns off power to the port, generates a syslog message, and updates the
power budget and LEDs.
On a Catalyst 3750-E switch, the PoE feature operates the same whether or not the switch is a stack
member. The power budget is per-switch and independent of any other switch in the stack. Election of
a new stack master does not affect PoE operation. The stack master keeps track of the PoE status for all
switches and ports in the stack and includes the status in output displays.
Power Management Modes
The switch supports these PoE modes:
• auto—The switch automatically detects if the connected device requires power. If the switch
discovers a powered device connected to the port and if the switch has enough power, it grants
power, updates the power budget, turns on power to the port on a first-come, first-served basis, and
updates the LEDs. For LED information, see the hardware installation guide.
If the switch has enough power for all the powered devices, they all come up. If enough power is
available for all powered devices connected to the switch, power is turned on to all devices. If there
is not enough available PoE, or if a device is disconnected and reconnected while other devices are
waiting for power, it cannot be determined which devices are granted or are denied power.
If granting power would exceed the system power budget, the switch denies power, ensures that
power to the port is turned off, generates a syslog message, and updates the LEDs. After power has
been denied, the switch periodically rechecks the power budget and continues to attempt to grant
the request for power.
If a device being powered by the switch is then connected to wall power, the switch might continue
to power the device. The switch might continue to report that it is still powering the device whether
the device is being powered by the switch or receiving power from an AC power source.
If a powered device is removed, the switch automatically detects the disconnect and removes power
from the port. You can connect a nonpowered device without damaging it.
You can specify the maximum wattage that is allowed on the port. If the IEEE class maximum
wattage of the powered device is greater than the configured maximum value, the switch does not
provide power to the port. If the switch powers a powered device, but the powered device later
requests through CDP messages more than the configured maximum value, the switch removes
power to the port. The power that was allocated to the powered device is reclaimed into the global
power budget. If you do not specify a wattage, the switch delivers the maximum value. Use the auto
setting on any PoE port. The auto mode is the default setting.
• static—The switch pre-allocates power to the port (even when no powered device is connected) and
guarantees that power will be available for the port. The switch allocates the port configured
maximum wattage, and the amount is never adjusted through the IEEE class or by CDP messages
from the powered device. Because power is pre-allocated, any powered device that uses less than or
equal to the maximum wattage is guaranteed to be powered when it is connected to the static port.
The port no longer participates in the first-come, first-served model.