Cisco Catalyst 3750-E Series, Руководство по настройке программного обеспечения

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12-8
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-08
Chapter 12 Configuring Interface Characteristics
Interface Types
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A Cisco pre-standard powered device does not provide its power requirement when the switch
detects it, so the switch allocates 15.4 W as the initial allocation for power budgeting.
The initial power allocation is the maximum amount of power that a powered device requires. The
switch initially allocates this amount of power when it detects and powers the powered device. As
the switch receives CDP messages from the powered device and as the powered device negotiates
power levels with the switch through CDP power-negotiation messages, the initial power allocation
might be adjusted.
•
The switch classifies the detected IEEE device within a power consumption class. Based on the
available power in the power budget, the switch determines if a port can be powered.
Table 12-1 lists
these levels.
The switch monitors and tracks requests for power and grants power only when it is available. The switch
tracks its power budget (the amount of power available on the switch for PoE). The switch performs
power-accounting calculations when a port is granted or denied power to keep the power budget up to
date.
After power is applied to the port, the switch uses CDP to determine the CDP-specific power
consumption requirement of the connected Cisco powered devices, which is the amount of power to
allocate based on the CDP messages. The switch adjusts the power budget accordingly. This does not
apply to third-party PoE devices. The switch processes a request and either grants or denies power. If the
request is granted, the switch updates the power budget. If the request is denied, the switch ensures that
power to the port is turned off, generates a syslog message, and updates the LEDs. Powered devices can
also negotiate with the switch for more power.
Note The CDP-specific power consumption requirement is referred to as the actual power consumption
requirement in the Catalyst 3750 and 3560 software configuration guides and command references.
If the switch detects a fault caused by an undervoltage, overvoltage, overtemperature, oscillator-fault, or
short-circuit condition, it turns off power to the port, generates a syslog message, and updates the power
budget and LEDs.
On a Catalyst 3750-E switch, the PoE feature operates the same whether or not the switch is a stack
member. The power budget is per-switch and independent of any other switch in the stack. Election of a
new stack master does not affect PoE operation. The stack master keeps track of the PoE status for all
switches and ports in the stack and includes the status in output displays.
Table 12-1 IEEE Power Classifications
Class Maximum Power Level Required from the Switch
0 (class status unknown) 15.4 W
1 4 W
2 7 W
3 15.4 W
4 (reserved for future use) Treat as class 0