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Operating Rules

External Switch Power

Switch Priority Class

Port priority classification can be used by the switch to allocate power to

ports. It is a prioritization scheme by which the user can assign a low (default),

high, or critical priority to any given port. This assignment is done through

the command line interface (see the software manual that came with your

switch) of the switch and alters the hardware port-number priority for power

allocation.

A certain amount of power is consumed from the switch to the powered device

(typically less than 16% loss), which can be influenced by cable length, quality,

and other factors. The IEEE 802.3af specification has addressed loss of power

by providing more power than a powered device requires. As well, depending

upon the classification (Class 0-3) of the device, the switch will provide more

or less power to address the specific power needs of that end device.

A PD is classified based on the maximum power it draws across all input

voltages and operational modes. The most common class is 0, in which the

switch will allow a maximum draw of 15.4 watts per port. As an example, 15.4

watts - Power Loss (16%) = 12.95 watts. See table 2-2.

2-6

Low - Default. This priority class receives power only if all other priority

classes are receiving power. If there is enough power to provision PDs on

only some of the ports with a low priority, then power is allocated to the

ports in ascending order, beginning with the lowest-numbered port in the

class until all available power is in use.

High - This priority class receives power only if all PDs on ports assigned

with a critical priority are receiving full power. If there is not enough

power to provision PDs on ports assigned with a "High" priority, then no

power goes to the low priority ports. If there is enough power to provision

PDs on only some of the "High" priority ports, then power is allocated to

the "High" priority ports in ascending order, beginning with lowest-

numbered high priority port, until all available power is in use.

Critical - This priority class always receives power. If there is not enough

power to provision PDs on all of the ports configured for this class, then

no power goes to "High or Low" priority ports. If there is enough power

to provision PDs on only some of the "Critical" ports, then power is

allocated to the "Critical" ports in ascending order, beginning with the

lowest-numbered port in the class.