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Chapter 1: Overview
Port
Prioritization
Wiring
Implementation
24
If the power requirements of the powered devices exceed the switch's
power budget, the switch denies power to some ports based on a system
called port prioritization. You may use this mechanism to ensure that
powered devices critical to the operations of your network are given
preferential treatment by the switch in the distribution of power should the
demands of the devices exceed the available capacity.
There are three priority levels:
Critical
High
Low
Ports set to the Critical level, the highest priority level, are guaranteed
power before any of the ports assigned to the other two priority levels.
Ports assigned to the other priority levels receive power only if all the
Critical ports are receiving power. Ports that are connected to your most
critical powered devices should be assigned to this level. If there is not
enough power to support all the ports set to the Critical priority level,
power is provided to the ports based on port number, in ascending order.
The High level is the second highest level. Ports set to this level receive
power only if all the ports set to the Critical level are already receiving
power. If there is not enough power to support all of the ports set to the
High priority level, power is provided to the ports based on port number, in
ascending order.
The lowest priority level is Low. This is the default setting. Ports set to this
level only receive power if all of the ports assigned to the other two levels
are already receiving power. As with the other levels, if there is not enough
power to support all of the ports set to the Low priority level, power is
provided to the ports based on port number, in ascending order.
Power allocation is dynamic. If the switch's power budget is at its
maximum, ports may stop supplying power to powered devices if new
powered devices, connected to ports with higher priorities, become active.
The IEEE 802.3af standard defines two methods for the delivery of DC
power over twisted pair cable by a PSE, such as the switch, to PDs. These
methods, known as modes A and B, identify the wires within the cable that
carry the DC power from the PSE to a PD.
Twisted pair cabling typically consists of eight wires. With 10BASE-T and
100BASE-TX devices, the wires connected to pins 1, 2, 3, and 6 on the
RJ- 45 connectors carry the network traffic while the wires connected to
pins 4, 5, 7, and 8 are unused. With 1000BASE-T devices, all eight wires
are used to carry network data.
It takes four wires to deliver DC power to a PD. With Mode A, the power is
delivered on pins 1, 2, 3, and 6. These are the same pins in 10BASE-T
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