Rockwell Automation 1783-BMxxx Stratix 5700 Ethernet Managed Switches User Manual - Manual (Page 88)

88 Rockwell Automation Publication 1783-UM004E-EN-P - June 2014
Chapter 3 Switch Software Features
• Static—The switch pre-allocates power to the port even when no powered
device is connected and guarantees that power is 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 a
powered Cisco end 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 a powered Cisco end device needs more than
the maximum wattage, the powered device is shut down.
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.
• Off—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 port, making
the port a data-only port.
Maximum Power Allocation (cutoff power) on a PoE Port
The switch determines the cutoff power on a PoE port in this order.
1. Manually when you configure the power level to budge for the port
2. Manually when you configure the power level that limits the power
allocated to the port
3. Automatically when the switch sets the power usage of the device by using
the IEEE classification and LLDP power negotiation or CDP power
negotiation
If you do not manually configure the cutoff-power value, the switch can
automatically determine the value by using CDP power negotiation when
connected to a Cisco end device. If the switch cannot determine the value by
using one of these methods, it uses the default value of 15.4 W.
With PoE+, if you do not manually configure the cutoff-power value, the switch
automatically determines it by using the device IEEE classification and LLDP
power negotiation or CDP power negotiation with a Cisco end device. If CDP or
LLDP are not enabled, the default value of 30 W is applied. However, without
CDP or LLDP, the switch does not allow devices to consume more than 15.4 W
of power because values from 15,400…30,000 mW are allocated based on only
CDP or LLDP requests. If a powered device consumes more than 15.4 W
without CDP or LLDP negotiation, the device can be in violation of the
maximum current limitation and can experience a fault for drawing more current
than the maximum. The port remains in the fault state for a time before
attempting to power on again. If the port continuously draws more than 15.4 W,
the cycle repeats.