Power over Ethernet on Cisco IP Phones
Power over Ethernet, and utilization planning against switch capacity is an important consideration for any IT department. In addition to being a useful tool for IT managers, inline power commands can be used in such a way that we can force a phone to reset from the switch device, without affecting normal operations of the port. This can be advantageous, for example, when a PC is connected to the network via the phone.
The power utilization for each Cisco IP phone can be found here. The 7945 and 7965s are missing from this list, however testing indicates that these are Class 3 devices that draw 12 Watts in normal operation.
Observe in the following output we have 3 IP phones connected - a 7941, a 7961 and a 7960.
VOICE-SWITCH# sh power inline
Module Available Used Remaining
(Watts) (Watts) (Watts)
------ --------- -------- ---------
1 370.0 19.6 350.4
Interface Admin Oper Power Device Class Max
(Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Fa1/0/3 auto on 6.3 IP Phone 7960 n/a 15.4
Fa1/0/4 auto on 7.0 IP Phone 7941 2 15.4
Fa1/0/5 auto on 6.3 IP Phone 7961 2 15.4
The Cisco 3560 and 3750 switches (which appear in the lab) provde some interesting customization options around inline power provision; we can:
- Remove inline power altogether
- Reduce the maximum consumption on a port
- Specify the actual known consumption for a port (this is for calculation of power to non-Cisco devices; i.e. those that can not advertise actual consumption using CDP)
The first option is great - by denying inline power to a port, and then re-allowing, the phone resets. Observe the following configuration, and impact to the switch:
interface fa 1/0/3
power inline never
VOICE-SWITCH#sh power inline
Interface Admin Oper Power Device Class Max
(Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Fa1/0/3 off off 0.0 n/a n/a 15.4
The phone no longer has nower. Specifying power inline auto (or no power inline never) provides power back to the phone, causing a hard reset of the IP phone. This can be handy when you don't have access to CallManager, but you want to reset the phone without affecting the connected PC.
Now, observe the effect of specifying a maximum power less than the IP phone actually requires; although the switch allocates 15.4 Watts for a 7960 when it first boots, it only requires 6.3 Watts to operate (and if this is the maximum specified under the port, the phone will actually still boot). First, observe the advertised power usage of the attached 7960 device, learned via CDP.
VOICE-SWITCH#sh cdp nei fa 1/0/3 det
-------------------------
IP address: 192.168.255.15
Platform: Cisco IP Phone 7960, Capabilities: Host
Interface: FastEthernet1/0/3, Port ID (outgoing port): Port 1
Power drawn: 6.300 Watts
Now, for observation purposes, we reduce the maximum PoE allowance to 4 Watts.
interface fa 1/0/3
power inline auto max 4000
VOICE-SWITCH#sh power inline
Module Available Used Remaining
(Watts) (Watts) (Watts)
------ --------- -------- ---------
1 370.0 28.6 341.4
Interface Admin Oper Power Device Class Max
(Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Fa1/0/3 auto on 4.0 Cisco PD n/a 4.0
The phone will now attempt to boot - the "headset" button will light, then the "mute" button, and the "speaker" button briefly, before power is lost.
The last feature of inline power in the Cisco switches is the ability to specify actual drawn power of a connected device. Normally, this is learned via CDP - however if you've gone down to the local electronics store to pick-up an 802.3af Class 3 SIP device, the switch will always budget the maximum Watts for the relevant class - i.e. 15.4 Watts. But what if the device actually only draws 7 Watts in normal operation? You've just cut your switch capacity in half! The command power inline consumption can be used to manually override the actual power consumption of a PoE device.