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28-13
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide
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Release 8.2GLX
78-15908-01
Chapter 28 Power Management
Understanding How PoE Works
Wall-Powered Phones
When a wall-powered phone is present on a switching module port, the switching module cannot detect
its presence. The supervisor engine discovers the phone through CDP messaging with the port. If the
phone supports PoE (the supervisor engine determines this through CDP), and the mode is set to Auto,
Static, or Off, the supervisor engine does not attempt to power on the port. If a power outage occurs, and
the mode is set to Auto, the phone loses power, but the switching module discovers the phone and
informs the supervisor engine, which then applies PoE to the phone. If a power outage occurs, and the
mode is set to Static, the phone loses power, but the switching module discovers the phone and applies
the preallocated PoE to the phone.
Powering Off the Phone
The supervisor engine can turn off power to a specific port by sending a message to the switching
module. The power for a port in Auto mode is then added back to the available system power. Power for
ports in Static mode is not added back to the available system power. This situation occurs only when
you power off the phone through the CLI or SNMP.
Phone Removal
The switching module informs the supervisor engine if a powered phone is removed using a link-down
message. The supervisor engine then adds the allocated power for that port back to the available PoE if
the port is in Auto mode.
In addition, the switching module informs the supervisor engine if an unpowered phone is removed.
Caution
When you plug a Cisco IP phone into a port and turn the power on, the supervisor engine waits 4 seconds
for the link to go up on the line. During this time, if you unplug the phone cable and plug in a network
device, you could damage the device. We recommend that you wait at least 10 seconds between
unplugging a device and plugging in a new device.
Cisco high-power powered device
15.4
Cisco Aironet 1200 Access Point with 802.11a
and 802.11b radio installed
11
Table 28-4 Power Requirements for Some Powered Devices (continued)
Device
Required Power (W)