23
Configuring Interfaces
Understanding Interface Types
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.
However, if the powered-device IEEE class is greater than the maximum wattage, the switch does not supply power
to it. If the switch learns through CDP messages that the powered device needs more than the maximum wattage,
the powered device is shutdown.
If you do not specify a wattage, the switch pre-allocates the maximum value. The switch powers the port only if it
discovers a powered device. Use the static setting on a high-priority interface.
never—The switch disables powered-device detection and never powers the PoE port even if an unpowered device
is connected. Use this mode only when you want to make sure power is never applied to a PoE-capable port, making
the port a data-only port.
For information on configuring a PoE port, see Configuring a Power Management Mode on a PoE Port, page 36.
Power Monitoring and Power Policing
When policing of the real-time power consumption is enabled, the switch takes action when a powered device consumes
more power than the maximum amount allocated, also referred to as the cutoff-power value.
When PoE is enabled, the switch senses the real-time power consumption of the powered device. The switch monitors
the real-time power consumption of the connected powered device; this is called power monitoring or power sensing.
The switch also polices the power usage with the power policing feature.
Power monitoring is backward-compatible with Cisco intelligent power management and CDP-based power
consumption. It works with these features to ensure that the PoE port can supply power to the powered device. For more
information about these PoE features, see Supported Protocols and Standards, page 21.
The switch senses the real-time power consumption of the connected device as follows:
1. The switch monitors the real-time power consumption on individual ports.
2. The switch records the power consumption, including peak power usage. The switch reports the information through
the CISCO-POWER-ETHERNET-EXT-MIB.
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