Management and Configuration Guide K/KA/KB.15.15

Power-sourcing equipment (PSE) detects the power needed by a powered device (PD) before
supplying that power, a detection phase referred to as "searching." If the PSE cannot supply the
required amount of power, it does not supply any power. For PoE using a Type 1 device, a PSE
will not supply any power to a PD unless the PSE has at least 17 watts available. For example, if
a PSE has a maximum available power of 382 watts and is already supplying 378 watts, and is
then connected to a PD requiring 10 watts, the PSE will not supply power to the PD.
For PoE+ using Type 2 devices, the PSE must have at least 33 watts available. A slot in a zl chassis
can provide a maximum of 370 watts of PoE/PoE+ power to a module.
Configuration options
In the default configuration, PoE support is enabled on the ports in a PoE module installed on the
switch. The default priority for all ports is low and the default power notification threshold is 80%.
Using the CLI, you can:
• Disable or re-enable PoE operation on individual PoE ports
• Enable support for pre-standard devices
• Change the PoE priority level on individual PoE ports
• Change the threshold for generating a power level notice
• Manually allocate the amount of PoE power for a port by usage, value, or class
• Allocate PoE power based on the link-partner’s capabilities via LLDP
NOTE: The ports support standard networking links and PoE links. You can connect either a
non-PoE device or a PD to a port enabled for PoE without reconfiguring the port.
PD support
To best utilize the allocated PoE power, spread your connected PoE devices as evenly as possible
across modules. Depending on the amount of power delivered to a PoE module, there may or may
not always be enough power available to connect and support PoE operation on all ports in the
module. When a new PD connects to a PoE module and the module does not have enough power
left for that port, if the new PD connects to a port "X" that has a:
Higher PoE priority than another port "Y" that is already supporting another PD, the power is
removed from port "Y" and delivered to port "X." In this case the PD on port "Y" loses
power and the PD on port "X" receives power.
Lower priority than all other PoE ports currently providing power to PDs, power is not supplied
to port "X" until one or more PDs using higher priority ports are removed.
In the default configuration (usage), when a PD connects to a PoE port and begins operating, the
port retains only enough PoE power to support the PD's operation. Unused power becomes available
for supporting other PD connections. However, if you configure the poe-allocate-by option
to either value or class, all of the power configured is allocated to the port.
For PoE (not PoE+), while 17 watts must be available for a PoE module on the switch to begin
supplying power to a port with a PD connected, 17 watts per port is not continually required if the
connected PD requires less power. For example, with 20 watts of PoE power remaining available
on a module, you can connect one new PD without losing power to any connected PDs on that
module. If that PD draws only 3 watts, 17 watts remain available, and you can connect at least
one more PD to that module without interrupting power to any other PoE devices connected to the
same module. If the next PD you connect draws 5 watts, only 12 watts remain unused. With only
12 unused watts available, if you then connect yet another PD to a higher-priority PoE port, the
lowest-priority port on the module loses PoE power and remains unpowered until the module once
again has 17 or more watts available.
124 Power Over Ethernet (PoE/PoE+) Operation