Verify Differences between PoE 802.3AT/802.3BT on Catalyst 9000

Introduction

This document describes the difference between PoE standards 802.3at and 803.bt.

Prerequisites

Requirements

Cisco recommends that you have knowledge of these topics:

• PoE (Power Over Ethernet)

Components Used

The information in this document is based on these software and hardware versions:

• Catalyst 9000 family and Line cards that supports PoE.
• Cisco IOS®

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Background Information

Terminology

• IEEE - Institute of Electrical and Electronics Engineers
• PoE - Power over Ethernet
• PoE+ - PoE+ standard increases maximum power that can be drawn by a powered device from 15.4 to 30W per port
• UPoE - Universal PoE. Cisco proprietary technology that extends the IEEE 802.3 PoE standard to provide the capability to source up to 60W of power over per port.
• CDP - Cisco Discovery Protocol, used to negotiate power between Cisco devices.
• LLDP - Link Layer Discovery Protocol used to negotiate power between Cisco and non Cisco devices

A PoE-capable switch port can provide power to one of the connected devices if the device senses that there is no power on the circuit. For this, three different standards were created in order to accomplish different tasks:

• An IEEE 802.3af-compliant powered device
• An IEEE 802.3at-compliant powered device
• An IEEE 802.3bt-compliant powered device

In 1999, IEEE standardized PoE to establish interoperability in a wide range of connected powered devices and power provider equipment. The first standards 802.3af specifies that the power must be able to be provided by either spare pairs (pins 4 and 5 or pins 7 and or data pairs (pins 1 and 2 or pins 3 and 6). Then, In 2009, IEEE 802.3at (PoE+ or Type 2) was created, It allows  us to increase power to 30W. Finally, in 2011 a new  Cisco proprietary standard came up that allows us to use all four twisted pairs, the IEEE 802.3bt standard defines 4PPoE Type 3 (UPOE) and let us provide up to 60W. Later on, in 2018 this standard allows us to increase the maximum power to 90W from the power source known as 4PPoE Type 4 (UPOE+).

Tu summarize:

The next section is focused on the 802.3at (Poe+) and 802.3bt (UPoe+) mode.

PoE Switch Models

• Catalyst 9000 switches and line cards with P in their product ID support PoE+ on a group of ports or all ports. For example, C9200L-48P-4G, C9200-24P, C9300-48P, C9400-LC-48P and so on.
• Catalyst 9000 switches and line cards with U in their product ID support UPoE on a group of ports or all ports. For example, C9300-24U, C9400-LC-48UX, and so on.
• Catalyst 9000 switches and line cards with H in their product ID support UPoE+ on a group of ports or all ports. For example, C9300-48H, C9400-LC-48H and so on.

Note:  PoE capability alone does not guarantee PoE assignment. The next section describes when CDP or LLDP is needed to negotiate the proper power.

This table describes the devices that support PoE+, UPOE and UPOE+:

Note: The Cisco Catalyst 9300 UPOE switches that support IEEE 802.3bt standard for Type 3 powered devices are in 802.3at mode, by default. 

Note: The Cisco Catalyst 9300 UPOE+ switches that support IEEE 802.3bt standard for Type 4 powered devices are in 802.3bt mode, by default. 

A switch classifies an IEEE compliant PoE device under a power consumption class and provides power as soon as the power device is detected.

IEEE 802.3at

• The PoE+ standard increases the maximum power that can be drawn by a powered device from 15.4 W per port to 30 W per port.
• The initial allocation for Class 0, Class 3, and Class 4 powered devices is 15.4 W. When a device starts up and uses CDP or LLDP to send a request for more than 15.4 W, it can be allocated up to the maximum of 30 W.
• The switch is standby until a PoE devices requests for power and grants power only when it is available. Then the switch verifies the power availability (the total amount of power available on the device for PoE) and performs a calculation when a port is provided or denied  of power  in order to keep the budget up to date. As soon as the device provides power, either CDP or LLDP is involved to negotiate the total amount of power that can be granted:

If CDP is involved during the negotiation to determine thepower consumption requirement of the connected Cisco powered devices, which is the amount of power to allocate based on the CDP messages. The switch adjusts the power budget accordingly. Note that CDP does not apply to third-party PoE devices. The switch processes a request and either grants or denies power. If the request is granted, the switch updates the power budget. If the request is denied, the switch ensures that power to the port is turned off,generates a syslog message, and updates the LEDs. Powered devices can also negotiate with the switch for more power.

With LLDP,  powered devices use media dependent interface (MDI) type, length, and value descriptions (TLVs), Power-via-MDI TLVs, for negotiation power up to 30 W. Cisco pre-standard devices and Cisco IEEE powered devices can use CDP or the IEEE 802.3at power-via-MDI power negotiation mechanism to request power levels up to 30 W.

• If no CDP/LLDP is supported on the PoE device, the command power inline port 2-event can be used in order to request for more than 15.4 W:

Switch(config)# interface Te1/0/1
Switch(config-if)# power inline port 2-event 

IEEE 802.3bt

• When IEEE 802.3bt mode is uded, Cisco UPOE devices function as 802.3bt Type 3 or Type 4 devices, and support up to Class 6 and class 8 respectively (refer the IEEE Power Classification table in the document) on every port.
• This is the procedure this standard states in order to allocate power:

      1. After device detection, the switch determines the device power requirements based on its type.

      2. The initial power allocation is the maximum amount of power that a powered device requires. The switch initially allocates this amount of power when it detects and powers the powered device.

      3. If the switch receives CDP messages from the powered device and as the powered device negotiates power levels with the switch through CDP power-negotiation messages, the initial power allocation can be adjusted. However, this is not needed as initially the switch allocates the maximum amount of power required, based on its class.

The switch classifies the detected IEEE device within a power consumption class. Based on the available power in the power budget, the switch determines if a port can be powered. 

• UPOE Devices are configured to use 802.3at mode, by default. To change the PoE standard to 803.bt mode, the command hw-module switch switch_noupoe-pluscommand can be used in the global configuration mode. A reload is needed:

Device# conf t
Device(config)#hw-module switch 1upoe-plus !!!WARNING!!!This configuration will power cycle the switch to make it effective. Would you like to continue y/n? Device# y

Device# show platform
Switch Ports   Model     Serial No.  MAC address    Hw Ver.  Sw Ver. 
------ ----- ---------  ----------- -------------- -------  --------
1        41  C9300-24UX FJB2318A04T 7802.b107.bf00   V02    17.03.05 
Switch/Stack Mac Address : 7802.b107.bf00 - Local Mac Address
! Output omitted for brevity

Device# show power inline Te1/0/24 detail
Interface: Te1/0/24
Inline Power Mode: auto
Operational status: on
Device Detected: yes
Device Type: Cisco IP Phone 7940
IEEE Class: n/a   <-- Type of class
Police: off

Power Allocated
Admin Value: 60.0
Power drawn from the source: 6.3
Power available to the device: 6.3

Actual consumption
Measured at the port: 1.9
Maximum Power drawn by the device since powered on: 1.9

Absent Counter: 0
Over Current Counter: 0
Short Current Counter: 0
Invalid Signature Counter: 0
Power Denied Counter: 0

Power Negotiation Used: CDP   <-- Protocol used to negotiate power
LLDP Power Negotiation --Sent to PD-- --Rcvd from PD--
Power Type: - -
Power Source: - -
Power Priority: - -
Requested Power(W): - -
Allocated Power(W): - -

Four-Pair PoE Supported: Yes   <-- Four pair copper support
Spare Pair Power Enabled: No   <-- Spair pair enabled
Four-Pair PD Architecture: N/A

Device# show power inline upoe-plus 
Module Available Used Remaining
(Watts) (Watts) (Watts) 
------ --------- -------- ---------
1 595.0 0.0 595.0

Device IEEE Mode - AT   <-- PoE standard used in the device

Codes: DS - Dual Signature device, SS - Single Signature device 
SP - Single Pairset device 
Interface Admin Type Oper-State Power(Watts) Class Device Name
State Alt-A,B Allocated Utilized Alt-A,B 
----------- ------ ---- ------------- --------- --------- ------- -----------
Te1/0/1 auto n/a off 0.0 0.0 n/a 
Te1/0/2 auto n/a off 0.0 0.0 n/a 

Related Information

• Technical Support & Documentation - Cisco Systems
• What Is Power over Ethernet (PoE)?
• Cisco Catalyst 9400 Series Switch Line Cards Data Sheet
• Cisco Catalyst 9200 Series Switches Data Sheet
• Cisco Catalyst 9300 Series Switches Data Sheet
• Cisco Catalyst 9400 Series Switch Data Sheet
• Interface and Hardware Components Configuration Guide (Catalyst 9300 Switches)
• Interface and Hardware Components Configuration Guide (Catalyst 9400 Switches)