Do the Catalyst 2960-S switches support Cisco pre-standard PoE?

[James Hawkins]

Hi,

I have a customer with Cisco 7940 and 7960 IP phones that they do not plan to replace. They do want a new LAN and are looking at the Catalyst

WS-C2960S-48FPD-L and WS-C2960S-24PD-L as access layer devices.

Please can someone confirm that these switches support the Cisco pre-standard PoE required by the 7940 and 7960 IP phones.

Thanks

James

[Ivan Shirshin]

hi James,

They do. Check the datasheet:

IEEE 802.3af and Cisco prestandard PoE  support comes with automatic discovery to detect a Cisco prestandard or  IEEE 802.3af endpoint and provide the necessary power without any user  configuration. Per-port PoE power sensing measures the actual power  being drawn, enabling more intelligent control of powered devices.

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/product_data_sheet0900aecd806b0bd8.html

But be aware of the DDTS which may causes PoE issues on 2960 as well:

     CSCta78591    3560/3750v2 POE switches not providing power to Cisco pre-standard phone

Symptom:

Catalyst 3560v2/3750v2 POE switches do not provide power to some Cisco pre-standard IP phones.

Conditions:

This issue is seen with Catalyst 3560v2/3750v2 switches and the following Cisco pre-standard IP phone models and revisions:

- Cisco CP-7940G IP phones with hardware revisions:

68-1735-05

68-1735-06

68-1735-07

68-1735-09.

68-1735-09 Rev. C0

- Cisco CP-7960G IP phones with hardware revisions:

68-1679-07 Rev. E0

68-1679-08 Rev. A0

Workaround:

Upgrade to an IOS version that integrates the fix for this problem

Kind Regards,
Ivan

**Please grade this post if you find it useful.

[James Hawkins]

Hi Ivan,

Thank you for the response. The switches that I am looking are the LAN Base rather than LAN Lite versions covered in the data sheet that you linked to.

The LAN Base data sheet (see link below) does not mention prestandard PoE

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/product_data_sheet0900aecd80322c0c.html

The PoE section of the data sheet states:

Power over Ethernet Plus PoE+

In addition to PoE 802.3af, the Cisco Catalyst 2960-S Series Switches support PoE+ (IEEE 802.3at standard), which provides up to 30W of power per port. The Cisco Catalyst 2960-S and 2960 Series Switches can provide a lower total cost of ownership for deployments that incorporate Cisco IP phones, Cisco Aironet^® wireless LAN (WLAN) access points, or any IEEE 802.3af-compliant end device. PoE removes the need for wall power to each PoE-enabled device and eliminates the cost for additional electrical cabling and circuits that would otherwise be necessary in IP phone and WLAN deployments.

As there is no mention of pre-standard PoE support does it mean that it is not supported?

[Ivan Shirshin · Accepted solution]

Hi James,

It actually meant by default it is supported, I think. I have checked the following 2 guides for this switches and it is said that:

1. PoE switch ports automatically supply power to these connected devices  (if the switch senses that there is no power on the circuit):

    •Cisco pre-standard powered devices (such as Cisco IP Phones and Cisco Aironet access points)

    •IEEE 802.3 af-compliant powered devices

    •IEEE 802.3 at-compliant powered devices (PoE+ on Catalyst 2960-S switches only)

    The switch detects a Cisco pre-standard or an IEEE-compliant powered  device when the PoE-capable port is in the no-shutdown state, PoE is  enabled (the default), and the connected device is not being powered by  an AC adaptor.

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

    •A  Cisco pre-standard powered device does not provide its power  requirement when the switch detects it, so a Catalyst 2960 switch  allocates 15.4 W as the initial allocation for power budgeting; a  Catalyst 2960-S switch allocates 30 W (PoE+).

    Catalyst 2960 and 2960-S Software Configuration Guide, 12.2(55)SE

    http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_55_se/configuration/guide/swint.html

    Note that this guide is valid for all 2960-S switches.

2. Cisco switches with PoE capability automatically supply power to connected pre-standard powered devices, such as Cisco IP phones and Cisco Aironet access points, and to IEEE 802.3af-compliant powered devices if the switch senses that there is no power on the circuit.

    http://www.cisco.com/en/US/products/hw/phones/ps379/products_qanda_item09186a00808996f3.shtml#qa2

There are some things to be aware of, though:

1. The switch might not support a pre-standard powered device—such as Cisco IP phones and access points that do not fully support IEEE 802.3af—if that powered device is connected to the switch through a crossover cable. This is regardless of whether auto-MIDX is enabled on the switch port.

2. A Cisco pre-standard powered device does not  provide its power requirement when the switch detects it, so a Catalyst 2960-S switch allocates 30 W (PoE+).

if you do not manually configure the cutoff-power  value, the switch automatically determines it by using CDP power  negotiation or the device IEEE classification and LLDP power  negotiation. If CDP or LLDP are not enabled, the default value of 30 W  is applied. However without CDP or LLDP, the switch does not allow  devices to consume more than 15.4 W of power because values from 15400  to 30000 mW are only allocated based on CDP or LLDP requests. If a  powered device consumes more than 15.4 W without CDP or LLDP  negotiation, the device might be in violation of the maximum current (Imax) limitation and might experience an Icut fault for drawing more current than the maximum. The port remains in  the fault state for a time before attempting to power on again. If the  port continuously draws more than 15.4 W, the cycle repeats.

Kind Regards,

Ivan

**Please grade this post if you find it useful.

[Leo Laohoo · Accepted solution]

WS-C2960S-48FPD
WS-C2960S-24PD

Yes, these two models can.  I have >250 units of 2960S (PoE) LAN Base in my network and they can support up to 30.0 PoE power.  The maximum I've ever come across using is only 20.0w.

[James Hawkins]

Thanks both for taking the time to answer - most appreciated