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Upgrading to IEEE 802.3bt PoE

IEEE 802.3bt Overview

The IEEE 802.3bt standard (PoE-bt) increases the amount of power that can be delivered to powered devices over PoE ports. PoE-bt can supply a maximum of 90 W of power by utilizing all four pairs of wire in a standard RJ-45 Ethernet cable.

What’s New in IEEE 802.3bt

The IEEE 802.3bt standard includes enhancements to existing PoE functionality such as power management, negotiation and classification. For more information on these features, see Understanding PoE on EX Series Switches.

Four Pair Standard

Previous IEEE PoE standards have used two pairs out of four pairs of twisted wire in an Ethernet cable to deliver power. The pairs used depend on the mode of PoE operation: mode A or mode B. In mode A, PoE delivers power on the same pairs used to deliver data (pair 1-2 and pair 3-6). Mode B separates the power delivery from data delivery by using the spare pairs for power (pair 4-5 and pair 7-8).

PoE-bt is the first IEEE standard to deliver power over all four pairs of wire. Pre-standard implementations that provided power over four pairs, such as PoE-4P, are replaced by this standard.

Power Class Levels

PoE standards define classes of powered devices based on the levels of power that they require. The IEEE 802.3bt standard introduces two new types of PoE powered devices, type 3 and type 4, which add an additional four power class levels (5 through 8). Type 3, which includes classes 5-6, can support up to 60 W of power, and type 4, which includes classes 7-8, can support up to 90 W.

Powered Device Signatures

Before the power sourcing equipment (PSE) can deliver power to a connected powered device (PD), it performs a series of checks on the PD. The first check is known as signature detection. The PSE uses a low voltage to measure the resistance of the PD. If the correct level of resistance is detected, the switch knows that the PD is capable of receiving power. In PoE-bt, signature detection is performed on each set of pairs: the data pair and the spare pair. The connected PD must present a valid signature for each pairset to show that it can accept 4-pair power.

The PSE then performs a connection check to determine if the PD is a single-signature or a dual-signature PD. Single-signature PDs have one signature that applies to both sets of pairs: the data pair and the spare pair. Dual-signature PDs support two signatures, one for each pairset. Dual-signature PDs can support two power channels on a single interface, and each channel can support a different power class.

LLDP Power Negotiation

IEEE 802.3bt supports enhanced power negotiation using Link Layer Discovery Protocol (LLDP). LLDP power negotiation enables the PSE to refine the power allocation to the PD. For example, a PD using LLDP can request a lower amount of power than it was allocated based on its class designation.

PoE-bt extends the set of fields in the LLDP protocol to allow the PSE and PD to exchange information about the maximum amount of power that the PSE has available. This is not an allocation, but can be used to inform the power request from the PD.


The IEEE 802.3bt standard introduces automatic class functionality. The auto-class feature allows the PSE to determine the actual maximum power drawn by the PD. The PSE measures the power consumption of the PD over a defined time period. Based on that measurement, the PSE sets the maximum power output for the PD.

PoE-bt Feature Support

PoE-bt supports the same features as previous versions of PoE, with the exception of the following, which are not supported:

  • Static power management mode

  • Maximum power configuration

  • Guard band

The configuration commands for unsupported features are available in PoE-bt. This is to support configuration of these features in a mixed Virtual Chassis that includes both PoE-at and PoE-bt members.

Table 1 explains the PoE-bt behavior when the commands are configured. For a complete list of PoE configuration commands and default settings, see PoE Configurable Options.

Table 1: Behavior for unsupported commands in PoE-bt


PoE-BT Behavior

management static

If static mode is configured, the firmware will be set to class mode.


If maximum power is configured, the configuration is ignored.


Value is fixed to 1 W. If another value is configured, the configuration is ignored.

Upgrading to PoE-BT

Upgrading to a Junos OS release that supports PoE-bt does not enable PoE-bt. You must explicitly enable PoE-bt by upgrading the PoE controller software.

To upgrade the PoE controller software to PoE-bt, use the following command:

Verifying the Upgrade

To verify that the upgrade was successful, check the PoE firmware version. The PoE firmware should be upgraded to 3.0 or higher for PoE-bt.

To check the firmware version, use the following command:

Example output before the upgrade:

Example output after the upgrade:

When you have verified the firmware version, you must reboot the switch to enable PoE-bt. After the reboot, verify that PoE-bt mode is in effect using the following command:

Rollback to PoE-AT

If you load a version of Junos OS that does not support PoE-bt, an alarm will be raised:

In this case, you must rollback the PoE firmware to PoE-at.

To rollback to PoE-at, use the following command:

Verify the rollback using the same procedure described in Verifying the Upgrade.