Understanding PoE on EX Series Switches

Classification

Classification is a process by which the power sourcing equipment (PSE) and powered device (PD) exchange information to dynamically determine the power allocation. The process begins when a PD is connected to a PoE interface and presents a class signature. The PoE IEEE standards define classes for devices based on the levels of power they require.

The PSE responds with a power allocation based on the class of the PD. If LLDP is enabled on the interface, the allocation can be adjusted using LLDP power negotiation. See LLDP Power Negotiation for more information.

Note:

Powered devices that are not IEEE-compliant might not present a class signature. These will be assigned a default class of 0.

Table 2 lists the classes of powered devices and associated power levels. Because of line loss, the power range of the powered device is less than the maximum power delivered at the PoE port for each class. Line loss is influenced by cable length, cable quality, and other factors and is typically less than 16 percent of the maximum power.

LLDP Power Negotiation

In class management mode, LLDP power negotiation can be used to refine the power allocation to the PD though an exchange of LLDP messages. For example, if the actual power requirement of the PD is a lower amount of power than it was allocated based on its class designation, the PSE can reduce the power allocation.

The negotiated power allocation will include some additional power guard to accommodate cable length. This additional allocated power is approximately 15 percent of the requested value and it can allocate the power in small increments. For devices that use LLDP power negotiation, the power reserved for the interface is always greater than the LLDP-requested power value by the external POE device.

LLDP power negotiation is enabled by default in class management mode for LLDP interfaces. On interfaces that are in class management mode but are not enabled for LLDP, the power allocation is determined solely by the class of the PD.

Note:

Starting in Junos OS Release 18.1R1, on EX2300 and EX3400 switches, once power is allocated based on LLDP power negotiation, LLDP power negotiation remains in effect, even if the interface link status goes off and on, or if the LLDP configuration is changed.

Note:

LLDP power negotiation is not supported on EX3200 and EX4200 (except EX4200 PX models) switches.

Maximum Power Configuration

In static management mode, you configure the maximum power allocation for each PoE interface. The PSE allocates this amount of power to the interface from the maximum PoE power budget for the switch or line card. For example, if you specify a maximum value of 8.0 W for ge-0/0/3, the PoE controller allocates 8.0 W for this interface out of the maximum power consumption. This amount is allocated to the interface irrespective of whether a powered device is connected to the interface or the connected powered device uses less power than 8.0 W.

Note:

Static management mode is not supported in PoE-bt.

Because of line loss, the power received by the powered device can be less than the power available at the PoE port. Table 3 shows the maximum power available at a PoE port and the resulting power guaranteed to the powered device.

Maximum PoE Power Budget on EX2200, EX2300, EX3200, EX3300, EX3400, EX4100, EX4100-F, EX4200, EX4300, and EX4400 Switches

The maximum PoE power budget on EX2200, EX2300, EX3200, EX3300, EX3400, EX4100, EX4100-F, EX4200, EX4300, and EX4400 switches depends on the switch model and the capacities of the power supplies installed. To find the maximum PoE power budget for each switch model, see Table 4 for EX2200 switch models, Table 5 for EX2300 switch models, Table 6 for EX3200 switch models, Table 7 for EX3300 switch models, Table 8 for EX3400 switch models, Table 9 for EX4100 and EX4100-F switch models, Table 11 for EX4200 switch models, Table 12 for EX4300 switch models, Table 13 for EX4400 switch models, and Table 14 for EX4400 switch models if Junos OS Release 22.2R1 or earlier is installed in the switch.

The maximum PoE power budget for a switch is displayed in the Maximum power field in the output of the show poe controller CLI command. The exception to this would be when LLDP power negotiation is in use.

If your switch supports power supplies of different capacities, keep the following points in mind:

• If you change your existing power supply to a lower-capacity power supply, the maximum PoE power budget might no longer be sufficient to power all the PoE ports on the switch.

• If your switch supports redundant power supplies and you have installed power supplies of different capacities, the maximum PoE power budget is based on the wattage of the lowest-capacity power supply.

• You cannot increase the number of PoE-capable ports on a switch by installing a power supply that has a higher capacity.

Table 4 lists the EX2200 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Table 5 lists the EX2300 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Table 6 lists the EX3200 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Table 7 lists the EX3300 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Table 8 lists the EX3400 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Table 9 lists the EX4100 and EX4100-F switch models, number of PoE-enabled ports on them, power supply ratings, and maximum PoE power budget.

Table 11 lists the EX4200 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Table 12 lists the EX4300 switch models, number of PoE-enabled ports, power supply ratings, and maximum PoE power budget.

Note:

EX4300 switches support power supply redundancy. For information on PoE power availability in N+N configurations and different PSU combinations, see AC Power Supply in EX4300 Switches.

Maximum PoE Power Budget on EX6200 and EX8200 Switches

For EX6200 and EX8200 switches, each line card that supports PoE has its own PoE controller and maximum PoE power budget. The maximum PoE power budget is allocated to the line card by the switch’s power management, while PoE power is allocated to the ports on the line card by the PoE controller. Because EX6200 and EX8200 switches can differ in the number and capacity of power supplies installed and in the number and types of line cards installed, the amount of power available for PoE power can vary for switches of the same model.

Power management allocates PoE power to line cards that support PoE only after it has allocated base power to and powered on all line cards. It then allocates the remaining power to the line cards for PoE in order of line card power priority. (In a default configuration, power priority is determined by the line card slot number, with slot 0 having the highest priority.) If the remaining power is insufficient to provide PoE power to all PoE line cards, a low-priority line card might receive no PoE power or partial PoE power.

By default, power management allocates enough PoE power to a line card to power all PoE ports at their maximum supported power. If the powered devices connected to that line card require less power than that, you can configure a smaller maximum PoE power budget for the line card. For example, power management normally allocates 915 W of PoE power to a 48-port PoE+ 20 Gbps (EX8200-48PL) line card. If the powered devices connected to that line card consume no more than a total of 250 W, you can set the maximum PoE power budget for the line card to 250 W. Doing so frees 665 W, which then can be used to fulfill the PoE power needs of lower-priority line cards.

You can also configure the power priority of the PoE ports on a line card. If power management is unable to allocate enough power to a line card to meet its maximum PoE power budget, the line card’s PoE controller turns off power to PoE ports in reverse priority order as required to meet the reduced power allocation.

Power management adjusts PoE power allocations as power availability and demand in a switch change. As a general rule, power management allocates power to power on line cards before it allocates PoE power. For example, if you add a line card and there is insufficient power available to power it on, power management reduces the PoE power it provides to line cards, starting with the lowest priority line card, until it frees enough power to power on the new line card. When power management reduces the maximum PoE power budget for a line card because of insufficient power, it logs a message in the system log.

Note that the actual power consumed by the powered devices does not affect power management’s power allocation for a line card. If you have set the maximum PoE power budget for a line card to 500 W, power management allocates 500 W even if the powered devices are consuming less power than that. Similarly, the maximum PoE power budget is not increased if you add additional powered devices—if the powered devices require more than the 500 W maximum that you have configured, lower-priority devices do not receive power.

You can display the switch’s power budget maintained by power management, including its PoE power allocations, by using the show chassis power-budget-statistics command. You can also display the maximum PoE power budget for each line card in a switch by using the show poe controller command.

For more information about how power management allocates power, including PoE power, see Understanding Power Management on EX Series Switches.