Choose the right IP phones and POE switches
Last year we learned a lesson about POE switches the hard way. Cisco had some nice new gigabit phones start shipping, and had been shipping gigabit POE switches to power them. One of our customers wanted gigabit ethernet to their desktop, so we specified the gig phones with the gig switches. Since all the switches were going to be in one closet, we recommended four 48 port POE switches, along with 120 POE phones. Everything was going great, until we started putting the phones out on the desktop.
(As a side note, even though the voice traffic over Ethernet is about 87kbps, it costs less to run a single drop to the desk that can power the phone and connect the workstation, since it would take twice as many ports in the closet and Cat5E drops to run 10/100 to the phone and gigabit to the desktop.)
Most of the phones booted right up, but some appeared to be dead. There are usually very few phones that arrive broken, so we tried plugging them directly into the switch. Some of them worked, some of them didn’t. Puzzled, the tech ssh’d into the switch to see what was being logged.
The switches were the 48 port Catalyst 3750 POE switches, and the handsets were the CP-7941G-GE’s, which have now been replaced by the CP-7945G phones. The CP-7941G-GE’s are gigabit speed versions of the CP-7941G phones; otherwise they are the exact equivalent. See if you can spot anything fishy about the combo from the datasheets. No? I didn’t either.
Here’s a hint from the phone datasheet:
The phone supports IEEE 802.3af PoE (the phone is a Class 3 device); 48 VDC is required; it can be supplied locally at the desktop using an AC-to-DC power supply (part number CP-PWR-CUBE-3=). Use of the power supply also requires ordering one of the AC country cords listed in Table 6.
From the switch datasheet:
Measured 100% Throughput Power Consumption
(with maximum possible PoE loads)
Cisco Catalyst 3750 Series
Total Output BTU
414 BTU/ hour
And finally from a technical note buried on this page:
Table 1 IEEE 802.3af PSE and Powered-Device Power Classifications
Minimum Power Levels Output at the PSE
Maximum Power Levels at the Powered Device
0.44 to 12.95W
0.44 to 3.84W
3.84 to 6.49W
6.49 to 12.95W
Reserved for future use
Treat as Class 0
Reserved for future use: A Class 4 signature cannot be provided by a compliant powered device
The problem occurred whenever the 29th phone was plugged into the switch, and the message was that the switch had hit the limit of its power budget! Now, if I buy a 48 port POE switch, I expect to have 48 ports that I can use for phones. I think most people would. This was an unexpected and unwelcome surprise.
However, the message on the switch was correct. Let’s do the math.
With the CP-7941G phone that we had been using for the last 2 years, 48 powered phones per switch was the case. It is an 802.3af Class 2 device, and requires 7 Watts max at the port. 7 Watts x 48 port = 336 Watts, which is well within the power budget of 370 Watts on almost every Cisco stackable switch, whether they have 24 or 48 ports.
The change in speed from 10/100 on the CP-7941 phone to 10/100/1000 on the CP-7941G-GE phone somehow caused Cisco to reclassify the new phone as a 802.3af Class 3 device. That requires maximum power of 15.4 Watts. If you divide the switch power budget of 370 Watts by 15.4 Watts, the result is exactly 24.
The reason 28 phones are supported is found in this FAQ:
Q. A Catalyst 3560 switch with 48 ports supports 370W. Because C7941G is a Class 3 device, it requires up to 15.4W. Can this be reduced to 7W so that the switch can power all 48 phones?
A. If Cisco Discovery Protocol (CDP) is enabled, there is no need to reduce the power requirement to 7W. The phone is classified as a Class 3 device when it first powers up, but after it powers up, CDP sets the desired power level on the 3560 to 7W. This allows the switch to support 48 ports of phones.
Note: If you use C7941G-GE, it is not possible to power all 48 phones. C7941G-GE usually draws 12.9W. The total power available is 370W, and for 48 ports, this evenly divides up to ~7.71W per port. In this case, the 3560 switch can only support 28 phones that draw 12.9W each.
So, whether the switch has 24 or 48 ports, the maximum quantity of gigabit speed Cisco phones that can be plugged into it is dependent on how much power each phone draws, which for gigabit phones means a maximum of 28. That rule holds true for every 2960, 2975, 3560, and 3750 series switch that Cisco ships. The only switches that can power 48 gigabit speed phones in a 48 port switch or switch blade are the 3750E, 4500, and 6500 series switches, which I will discuss some other time.
Another handy FAQ answer from the same source:
Q. What are the power requirements for the various models of the IP phone models?
CP-7906G (5W) (Class 2)
CP-7911G (5W) (Class 2)
CP-7941G (6.3W) (Class 2)
CP-7941G-GE (12.9W) (Class 3)
CP-7961G (6.3W) (Class 2)
CP-7961G-GE (12.9W) (Class 3)
CP-7970G (10.25W) (Class 3)
CP-7971-G-GE (15.4W) (Class 3)
CP-7985G (12.55W) (Class 0, Not full brightness)
IEEE 802.3af Device — Class 0 (15.4W)
IEEE 802.3af Device — Class 1 (4W)
IEEE 802.3af Device — Class 2 (7W)
IEEE 802.3af Device — Class 3 (15.4W)
So, what did we do? What could we do? We admitted our mistake to the customer, worked with our great Cisco account manager and our wonderful distributor, and got the four 48 port gig switches swapped out for eight 24 port gig switches at the same exact price. We got the exchange turned around in a day, worked late and extra hard, and got the system rolled out on time.
Lesson learned, again.
Should we have known this before we specified the system? Maybe — that’s a tough question. The information is definitely in the datasheets, but it could certainly be clearer. I just keep telling myself that the accumulation of experience from mistakes like this one turns into wisdom at some point.
Author: Rolf Versluis
Adcap Network Systems — Atlanta and Miami
Great Local Engineers Creating Systems that Work!
Posted at Adcap Tech Tips