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Software Configuration Guide—Release 12.2(25)SG
OL-7659-02
Chapter8 Configuring Power over Eth ernet
This example shows how to set the PoE consumption to 5000 milliwatts for Fast Ethernet interface 4/1
regardless what is mandated by the 802.3af class of the discovered device, o r by any CDP packet
received from the powered device:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface fastethernet 4/1
Switch(config-if)# power inline consumption 5000
Switch(config-if)# end
Switch#
This example shows how to verify the PoE consumption for a given interface:
Switch# show power inline fastethernet 4/1
Available:677(w) Used:11(w) Remaining:666(w)
Interface Admin Oper Power(Watts) Device Class
From PS To Device
--------- ------ ---------- ---------- ---------- ------------------- -----
Fa4/1 auto on 11.2 10.0 Ieee PD 0
Interface AdminPowerMax AdminConsumption
(Watts) (Watts)
---------- --------------- --------------------
Fa4/1 15.4 10.0
Switch#
Intelligent Power Management
All Catalyst 4500 PoE-capable modules use Intelligent Power Management to provision power on each
interface. When a powered device (PD) is attached to a PoE-capable port, the port will detect the PD and
provision power accordingly. If a Cisco PD is used, the switch and PD negotiate power using CDP
packets to determine the precise amount of power needed by the PD. If the PD is 802.3af compatible,
the difference between what is mandated by the 802.3af cl ass and what is actually needed by the PD is
returned to the power budget for use by additional devices. In this way, power negotiation al lows
customers to stretch their power budget and use it more effectively.
Power negotiation also enables the interoperability of newer Cisco powered devices with older legacy
PoE-capable ports from Cisco. Newer Cisco PDs do not consume more than what the switch port can
provide.
PoE and Supported Cabling Topology
When using PoE, pairs 2 and 3 (pins 1, 2, 3, and 6) of the fou r pairs in a standard UTP cable are used
for both the Ethernet data signals and the DC power at the same time. In DC, PoE flows from pair 3 (pins
3 and 6) to the device using PoE and back to pair 2 (pins 1 a nd 2) while the Ethernet port transmits
differential signals in pair 2 (between pins 1 and 2). This method of supplying DC power is sometimes
called “phantom power” because the power signals travel over the same two pairs used to transmit
Ethernet signals. The inline power signals are transparent to the Ethernet signals and do not interfere
with each other. The main electrical parameter that affects inline power operation and performance is the
DC resistance of the cable. The inline power method is designed t o work with category 3 cable and
above, up to 100 meters.
PoE has been tested and found to work with the IBM Token Ring STP cable (100 meters ) when used
with a Token Ring to Fast Ethernet adapter.