Cisco SWITCH Campus VoIP Refresh. Part 1 – POE

Support for IP Phones is provided by all modern Cisco Catalyst switches.

The normal functions required to support the phones are:

PoE

This review of PoE goes far beyond the knowledge required for SWITCH 642-813

Standards

IEEE 802.3af known as PoE and providing up to 15.4W per port.
IEEE 802.at known as PoE+ able to provide up to 30W per port.

PoE on Switches which carry this feature is usually on by default.
To review your switches interfaces status for power use commands :

SW3#show power inline 
Available:370.0(w) Used:0.0(w) Remaining:370.0(w)
Interface Admin Oper Power Device Class Max
 (Watts) 
--------- ------ ---------- ------- ------------------- ----- ----
Fa0/1 auto off 0.0 n/a n/a 15.4 
Fa0/2 auto off 0.0 n/a n/a 15.4 
Fa0/3 auto off 0.0 n/a n/a 15.4 
Fa0/4 auto off 0.0 n/a n/a 15.4 
Fa0/5 auto off 0.0 n/a n/a 15.4 
<truncated for brevity>

Also to view consumption configuration

SW3#show power inline consumption 
Interface Consumption Admin 
 Configured Consumption (Watts) 
---------- ----------- -------------------
Fa0/1 NO 0.0
Fa0/2 NO 0.0
Fa0/3 NO 0.0
Fa0/4 NO 0.0
Fa0/5 NO 0.0
<truncated for brevity>

You can remove PoE from an interface by using the interface command

SW3(config)#int fa 0/1
SW3(config-if)#power inline never
SW3(config-if)#do sh power inline
Available:370.0(w) Used:0.0(w) Remaining:370.0(w)
Interface Admin Oper Power Device Class Max
 (Watts) 
--------- ------ ---------- ------- ------------------- ----- ----
Fa0/1 off off 0.0 n/a n/a 15.4 
Fa0/2 auto off 0.0 n/a n/a 15.4 
<truncated for brevity>

You can now see that the Fa0/1 interface will never provide power.

When PoE is enabled, the switch senses the real-time power consumption
of the powered device. The switch monitors the real-time power consumption
of the connected powered device; this is called power monitoring or power sensing.
The switch also polices the power usage with the power policing feature.

One option is to limit the power sensing budget globally, or on a per port basis

By using the power inline consumption wattage configuration command,
you can override the default power requirement specified by the IEEE classification.
The difference between what is mandated by the IEEE classification and what is actually
needed by the device is reclaimed into the global power budget for use by additional devices.
You can then extend the switch power budget and use it more effectively.

For example, if the switch budgets 15,400 milliwatts on each PoE port as it does by default (3560/3750),
you can connect only 24 Class 0 powered devices. If your Class 0 device power
requirement is actually 5000 milliwatts, you can set the consumption wattage to
5000 milliwatts and connect up to 48 devices. The total PoE output power available
on a 24-port or 48-port 3560/3750 PoE switch is 370,000 milliwatts.

Globally:

SW3(config)#power inline consumption default 5000
%CAUTION: Misconfiguring the 'power inline consumption/allocation'
 command may cause damage to the switch and void your warranty. Take
 precaution not to oversubscribe the power supply. Refer to documentation.

Strangely, there doesn’t seem to be a show command which you can see that this global command is in effect.
Neither these commands have a different output as a result of this command being configured.

SW3#show power inline 
SW3#show power inline consumption

Whereas, the per interface configuration below shows up in the command

SW3#show power inline consumption 
Interface Consumption Admin 
 Configured Consumption (Watts) 
---------- ----------- -------------------
Fa0/1 NO 0.0
Fa0/2 YES 5.0
Fa0/3 NO 0.0

Per Interface, you can configure your ports as such

SW3(config-if)#power inline consumption 5000 
%CAUTION: Interface Fa0/2: Misconfiguring the 'power inline
 consumption/allocation' command may cause damage to the switch and void
 your warranty. Take precaution not to oversubscribe the power supply.
 It is recommended to enable power policing if the switch supports it.
 Refer to documentation.

As noted in the warning from the above budget modifications, you should enable power policing to prevent drawing so much power that you cause your switch to fail.

You can do this only on a per interface basis, so I’d recommend an interface range command on all your access ports

SW3(config)#interface range fastethernet 0/1 - 48
SW3(config-if-range)#power inline auto max 5000

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