Budgeting Cisco PoE for Aastra SIP Phones

I recently implemented a 45 phone FreeSWITCH system (if Asterisk is a mean and unforgiving Chef Ramsay, FreeSWITCH is Paula Deen, but that's for another post). With this number of handsets I definitely wanted to use a Power-over-Ethernet (PoE) switch feed them rather than having wall-warts locally at each station.

We already had a 48-port Cisco 3750-24PS at another location which was only powering a few access points, so we racked it up for its new job powering Aastra phones. Most of the phones we had decided on were the Aastra 57i with a couple 53i phones at rarely used stations.

Power Classification and Budget

Every PoE device, when attached to a PoE has the opportunity to communicate a power class back to the switch, this class tells the switch how many Watts the device needs in order to operate. Class 1 is 4W, Class 2 is 7W, and Class 3 is 15.4 Watts. This classification communication system is needed because reading actual current usage is impractical and because the switch needs to know that it will still be in budget before actually providing power.

This particular switch has a total power budget of 370 Watts, or an average of 7.7W per port. Think of the switch's power budget as a rope that can hold 370 pounds and each device's power requirement adds that much weight to the load. If you overload your rope IT WILL BREAK.

Now the problem with the Aastra phones is that they don't communicate their PoE class back to the switch. If the device doesn't communicate its class then it is classified as Class 0 (Unclassified) and budgeted the max of 15.4 W. We quickly max out the estimated power budget with only 24 phones, and once that happens the switch will refuse power requests for subsequent devices.

Not only do the phones not communicate with the switch, their tech specs, admin guide, and user's manual didn't communicate with me; I needed to know how many Watts the phones really required. To answer that question I had to get in touch with an Aastra reseller who opened a ticket with Aastra's support group. The answer? 4.5 Watts for the 57i.

Elucidating the Hardware

At 4.5W we can feel comfortable providing power on all 48 ports of this 3750 switch. The problem though is that the switch doesn't share our surety. So we need to find a way to share our newfound knowledge with Cisco IOS. Luckily Cisco has rather comprehensive documentation on their products, we'll take a look at Budgeting Power for Devices Connected to a PoE Port.

Turns out that the switch has a nifty command power inline consumption default that lets us tell it to ignore the IEEE classification and use a hard-coded number of milliwatts for a ports power budget. Since this command takes values in milliwatts (mW) we need to multiply our values by 1000; 4,500mW per phone and a total budget of 370,000mW.

Since at 4.5W we'll be quite under our power budget even when using 48 ports we're just going to tell the switch to budget 370,000/48 = 7700mW per port by default.

So simply log into your switch, get into enabled mode and execute these commands:

sac-switch-01#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
sac-switch-01(config)#power inline consumption default 7700
sac-switch-01(config)#end

It's that easy, but please heed the warning that you get when you use the power inline consumption default command. "Breaking your rope" may have been a metaphor, but "The switch is on fire!" will be an aphorism to your client!

%CAUTION: Interface interface-id: 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.

Don't forget to save out your running config copy running-config startup-config.

The Payoff

Great! We're done, and now running show power inline will show us how the switch is budgeting power, here's mine with about 30 of the phones attached:

Interface Admin  Oper       Power   Device              Class Max
                            (Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Fa3/0/1   auto   off        0.0     n/a                 n/a   15.4
Fa3/0/2   auto   on         7.7     Ieee PD             0     15.4
Fa3/0/3   auto   on         7.7     Ieee PD             1     15.4
Fa3/0/4   auto   on         7.7     Ieee PD             0     15.4
Fa3/0/5   auto   on         7.7     Ieee PD             0     15.4
Fa3/0/6   auto   on         7.7     Ieee PD             0     15.4
Fa3/0/7   auto   on         7.7     Ieee PD             1     15.4
Fa3/0/8   auto   on         7.7     Ieee PD             1     15.4
Fa3/0/9   auto   on         7.7     Ieee PD             0     15.4
Fa3/0/10  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/11  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/12  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/13  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/14  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/15  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/16  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/17  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/18  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/19  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/20  auto   on         7.7     AIR-AP1121G-A-K9    n/a   15.4
Fa3/0/21  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/22  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/23  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/24  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/25  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/26  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/27  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/28  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/29  auto   on         7.7     Ieee PD             0     15.4
Fa3/0/30  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/31  auto   on         7.7     Ieee PD             1     15.4
Fa3/0/32  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/33  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/34  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/35  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/36  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/37  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/38  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/39  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/40  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/41  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/42  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/43  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/44  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/45  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/46  auto   off        0.0     n/a                 n/a   15.4
Fa3/0/47  off    off        0.0     n/a                 n/a   15.4
Fa3/0/48  off    off        0.0     n/a                 n/a   15.4