Understanding PoE on EX Series Switches

PoE, PoE+, and Enhanced PoE

PoE was first defined in the IEEE 802.3af standard. In this standard, the amount of power that can be supplied to a powered device is limited to 15.4 W. A later standard, IEEE 802.3at, defined PoE+, which increases the amount of power to 30 W. The PoE+ standard provides support for legacy PoE devices—an IEEE 802.3af powered device can operate normally when connected to IEEE 802.3at (PoE+) power sourcing equipment.

Beginning in Juniper Networks Junos operating system (Junos OS) Release 11.1, Juniper Networks provides enhanced PoE on EX3200 and EX4200 switches. Enhanced PoE is a Juniper Networks extension to the IEEE 802.3af standard that allows power of up to 18.6 W per PoE port.

Table 1 lists EX Series switches and line cards and the version of PoE they support.

Note: This topic and its related topics use the term PoE as a generic term to refer to PoE, PoE+, and enhanced PoE.

PoE Power Allocation

A switch or line card that supports PoE has a PoE controller that keeps track of the PoE power consumption on the switch or line card, and allocates power to the PoE ports. The following factors determine how the PoE controller allocates power to the PoE ports:

• PoE Power Budget
• PoE Interface Power Allocation
• PoE Interface Power Priority

PoE Power Budget

The PoE power budget is the total amount of power available for the PoE controller to allocate to the PoE ports. The PoE controller cannot exceed its PoE power budget and does not allocate power to a PoE port if the allocation would exceed the PoE power budget.

How the PoE power budget is determined depends on the switch model:

• PoE Power Budget on EX2200, EX3200, EX3300, and EX4200 Switches
• PoE Power Budget on EX6200 and EX8200 Switches

PoE Power Budget on EX2200, EX3200, EX3300, and EX4200 Switches

The PoE power budget on EX2200, EX3200, EX3300, and EX4200 switches depends on the switch model and the capacities of the power supplies installed. To find the PoE power budget for each switch model, see Table 2 for EX2200 switch models, Table 3 for EX3200 switch models, Table 4 for EX3300 switch models, and Table 5 for EX4200 switch models.

Use the show poe controller command to display a switch’s PoE power budget.

• If you change your existing power supply to a lower-capacity power supply, the 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 PoE power budget is based on the wattage of the lower-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 2 lists the EX2200 switch models, number of PoE-enabled ports, power supply ratings, and PoE power budgets.

Table 3 lists the EX3200 switch models, number of PoE-enabled ports, power supply ratings, and PoE power budgets.

Table 4 lists the EX3300 switch models, number of PoE-enabled ports, power supply ratings, and PoE power budgets.

Table 5 lists the EX4200 switch models, number of PoE-enabled ports, power supply ratings, and PoE power budgets.

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 PoE power budget. The 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 different 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 PoE power budgets of PoE line cards 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 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 PoE power budget for the line card to 250 W. Doing so frees up 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 PoE power budget, the line card PoE controller will turn 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 changes. 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 up enough power to power on the new line card. When power management reduces the 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 PoE 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 PoE power budget is not increased if you add additional powered devices: if the powered devices require more than the 500 W PoE budget 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 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.

PoE Interface Power Allocation

You can configure how the switch determines the maximum power for a PoE interface and how power is allocated to the PoE interfaces. If the power consumption of a powered device exceeds the maximum power allocated to the interface, the switch turns off power to the interface.

These PoE power allocation methods are available:

• LLDP Power Negotiation
• Class PoE Management Mode
• Static PoE Management Mode

LLDP Power Negotiation

Link Layer Discovery Protocol (LLDP) power negotiation allows the PoE controller to dynamically allocate power to LLDP-enabled powered devices based on their power needs. The PoE controller allocates to an interface only the power currently required by the connected powered device, and it can allocate the power in small increments.

When the PoE management mode is set to class and LLDP is enabled (both are default settings), LLDP power negotiation is enabled by default. If you disable LLDP power negotiation or the powered device does not support it, the switch uses the class of the powered device to determine the maximum power for interfaces.

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

Class PoE Management Mode

In class PoE management mode, the maximum power for an interface is determined by the class of the connected powered device. Table 6 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, quality, and other factors and is typically less than 16 percent of the maximum power.

The powered device communicates to the PoE controller which class it belongs to when it is connected. The PoE controller then allocates to the interface the maximum power required by the class (see Table 6). It does not allocate power to an interface until a powered device is connected. Class 0 is the default class for powered devices that do not provide class information. Class 4 powered devices are supported only by PoE ports that support IEEE 802.3at (PoE+).

By default, when class PoE management mode and LLDP are enabled, LLDP power negotiation is also enabled on supported switches. See LLDP Power Negotiation for more information.

Static PoE Management Mode

In the static PoE management mode, you specify the maximum power for each PoE interface. The PoE controller then allocates this amount of power to the interface from its total budget. For example, if you specify a maximum value of 8.0 W for ge-0/0/3, the PoE controller allocates 8.0 W out of its total power budget for this interface. This amount is allocated to the interface whether or not a powered device is connected to the interface or whether the connected powered device uses less power than 8.0 W.

Because of line loss, the power received by the powered device can be less than the power available at the PoE port. Table 7 shows the maximum power available at a PoE port and the resulting power guaranteed to the powered device.

PoE Interface Power Priority

You can configure a PoE interface to have either a high or a low power priority. The power priority determines which interfaces receive power if PoE power demands are greater than the PoE power budget. If the total power allocated for all interfaces exceeds the switch budget, PoE power to lower-priority interfaces is turned off and the power allocated to those interfaces drops to 0. Thus you must set interfaces that connect to critical powered devices, such as security cameras and emergency phones, to high priority.

Among PoE interfaces that have the same assigned priority, power priority is determined by the port number, with lower-numbered ports having higher priority.

For EX6200 and EX8200 switches, interface power priority determines the relative priority of the interfaces on a line card, not on the switch as a whole. The relative priority of interfaces residing on different line cards is determined by line card priority. For example, if line card 1 has a higher power priority than line card 2 and a power shortage occurs, power is removed from the PoE interfaces in this order:

• Low-priority interfaces on line card 2
• High-priority interfaces on line card 2
• Low-priority interfaces on line card 1
• High-priority interfaces on line card 1

You can manually configure PoE interface power priority, or you can enable LLDP power priority, which assigns each interface the power priority provided by the connected LLDP-enabled powered device. Table 8 describes how the switch converts LLDP power priorities to switch power priorities.

Note: LLDP power priority requires LLDP power negotiation to be enabled, which is enabled by default when the PoE management option is set to class.

Note: LLDP power priority is not supported on EX3200 and EX4200 (except EX4200 PX model) switches.