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Saturday, June 16, 2012

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

We 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.
Can you please confirm that these switches support the Cisco pre-standard PoE required by the 7940 and 7960 IP phones.

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+).

Please click here for Catalyst 2960 and 2960-S Software Configuration Guide, 12.2(55)SE

    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. Please click here for Power over Ethernet (PoE) Power Requirements FAQ.

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.

Citation - This blog post does not reflect original content from the author. Rather it summarizes content that are relevant to the topic from different sources in the web. The sources might include any online discussion boards, forums, websites and others.

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