Inline power

antonyice1

i was asked in an interview that if i have a 48 port inline power cisco switch will i be able to provide power to all those 48 phones connected to that switch.My answer was "yes" but the guy who interviewed me said that we can't do that because of some power calculation that we should do using cisco "power calculater".could somebody explain about this .

nice343

the guy does not know how to ask a question. Technically you are right but his framing of the question gets a 1/10 from me

networker050184

A switch has about 370 Watts available for PoE. Each phone uses a max of 15.4W, but most use around 7W. You can do the math to see how many phones can be hooked up.

dtlokee

The answer depends on the model. The 4500's can provide power to all the interfaces on a 10/100/1000 blade at 15.4 watts each with no troube. The 3550/60 and 3750 can typically only provide power to 24 of their interfaces (assuming 15.4 watts) or all of them at 7 watts (like networker said many of the phones use 7 watts)

bighornsheep

The question is a good question, but asked in a way that may mislead the candidate, I think the interviewer basically wants to know that you understand PoE doesn't just mean power for all ethernet ports, there're different power consumption levels, there are different power supplies that will cater differently to those power levels.

dtlokee wrote:

The 3550/60 and 3750 can typically only provide power to 24 of their interfaces (assuming 15.4 watts) or all of them at 7 watts (like networker said many of the phones use 7 watts)

I believe the enterprise edition (3560E) has an option for a higher power supply which will supply 15.4 to all 48 ports.

antonyice1

ok so how do i configure my 3550 if i want to power up all my 48 ports to supply power to 48 ip phones

dtlokee

You can let CDP figure out how much the device needs or you can set it manually

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_25_see/configuration/guide/swint.html#wp1138642

antonyice1

dtlokee,

i didn't understand what u mean.how will cdp help us in this ?

networker050184

When the phone powers up the switch uses CDP to negotiate the proper power level with the phone.

mgeorge

antonyice1 wrote:

ok so how do i configure my 3550 if i want to power up all my 48 ports to supply power to 48 ip phones

Correction: Their are no 3550 switches that have 48 ILP ports.

The 3550's uses the Cisco's "inline power" aka ILP, by default. This is a Cisco designed
PoE standard, designed before the IEEE issued an industry standard (802.3af) the 3550's
CANNOT be upgaded to use the 802.3af standard.

3560; which uses the IEEE PoE 802.3af standard by default but is backwards compatiable with ILP

The 3560-48-PS-X have a 530WATT PSU and only dedicate 370 WATTS to PoE
The 3560 will only power 24 ports at max 15.4watts (Class 3 PoE)
But the 3560 WILL power 48 class 2 phones at 7 watts (Class 2 PoE)

Also the 3750 follows the same specifications at the 3560's. 24/48 c3/c2 PoE

4500's and 6500's will power every port in the chassis and then some.

Cisco 7961G's are class 2 phones and you can run 48 phones on a 3560 or 3750.

Most of the phones with color LCD screens are Class 3's so only 24 of those

Whenever you are asked a question like that in an interview, ask them to describe
the scenario better and dont take a guess on it because it could cost you the job.

Added: The -F models of 3560E and 3750E can support 48 class 3's

networker050184

I know this is a little late but I came across the Cisco Power Calculator while looking something up today incase anyone is interested.