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New Member

3560 with 48 PoE phones

I have a question on how a 3560 48-port PoE switch will behave in the event of a reload, if all end devices are say phones.

When the switch reloads it will only be able to provide full power to the first 24 devices. The 25th device probably get rejected for power. Once these 24 devices have loaded successfully and are using 7.7W power, what will happen to the 25th device. Will the phone automatically re-request power from the switch. If the original request for power is relay based, will that mean that human intervention will be needed to activate all other devices.

Is this correct !!!

Can I get my hands on any CCO documentation that explains this.

Thanks.

11 REPLIES

Re: 3560 with 48 PoE phones

I thought all the new switches do keep track of how much power is allocated per port. So I think it should allow 7.7 W per port for all 48 ports.

Blue

Re: 3560 with 48 PoE phones

I just checked a 3650 with 19 phones, and it shows 370 watts available, which would be enough power for all 48 ports at 7.7W. The phones (7940 and 7960) use 6.3 watts, so there is a big extra power margin if you just have IP phones.

AN-J2Users-sc3560#sh power inline

Available:370.0(w) Used:119.7(w) Remaining:250.3(w)

Interface Admin Oper Power Device Class Max

(watts)

--------- ------ ---------- ------- ------------------- ----- ----

Fa0/1 auto on 6.3 IP Phone 7940 n/a 15.4

Re: 3560 with 48 PoE phones

I dont think there is a command equivalent to the "set inlinepower defaultallocation " for the newer IOS based switches (3750,3560) because they keep track of how much power is allocated. It should automatically provide power to all 48 ports, if they are using 7.7 watts per port.

Super Bronze

Re: 3560 with 48 PoE phones

I think what Rafiq is asking is what will happen when all devices boot up simultaneously - in 802.1af (and I assume with Cisco Power) when devices start up they take the full 15.4 watts, then negotiate what they actually need and adjust appropriately.

So if all phones are started at once, then 15.4w * 48 ports = more power than is available. So is it just a matter of some ports not powering up straight away?

I've never had a switch fully loaded so I don't have the answer :-)

Aaron

Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
New Member

Re: 3560 with 48 PoE phones

Aaron is right, what would actually happen when you reload the switch, which will effectively reboot all the 48 phones.

Any suggestions, or does anyone have 25 or more plugged into a 3560 to do some testing.

Cheers.

Blue

Re: 3560 with 48 PoE phones

I have a 3560 with 38 phones, that has been power cycled with that load, and it comes up fine with no intervention. Perhaps it works through the ports sequentially doing power negotiation at startup?

New Member

Re: 3560 with 48 PoE phones

David,

Thanks for that.

Has anyone seen any formal Cisco documentation that describes the startup process.

Cheers.

Re: 3560 with 48 PoE phones

Read this link.

Cisco has also extended the concept of intelligent power management to IEEE 802.3af devices to better manage power resources within the switch. As an example a Cisco Aironet 1200 Access Point requires 8W for normal operation (Table 3), but is classified as a Class 3 device that has a class maximum of 15.4W. To provide more granular power management, Cisco supports the concept of power negotiation using Cisco Discovery Protocol (CDP) between a Cisco PSE and Cisco powered devices such that unused power can be returned to the Cisco PSE IPM budget. Using a Cisco Aironet 1200 Access Point, by explicitly negotiating that it only requires 8W instead of the Class 3 maximum of 15.4W, 7.4W is returned to the PSEs power budget thereby providing better power management granularity than the power classifications described within IEEE 802.3af-2003.

http://www.cisco.com/en/US/netsol/ns340/ns394/ns147/ns412/networking_solutions_white_paper09186a008026641c.shtml

New Member

Re: 3560 with 48 PoE phones

This is good, but what David was intimating at on "how the 3560 switch behaves" is very plausible.

I want to get my hands on this information from CCO if possible.

Cheers.

Re: 3560 with 48 PoE phones

Rafiq,

We have a CME deployment for about 60 phones and got the same exact equipment 3560 with POE. Tested 45 phones today on one 3560, plugged them all in and powered down the switch. Powered it back on, about half the phones got power immediately while half of them didnt and after a couple of minutes those phones started getting power. The reason being each port will be allocated 15.4 watts by default and then they will negotiate power and because each port was using only 6.3 watts the rest of the power goes back to the PSE power pool. Eventually all phones are up and running, just that if a power outage happens and power comes back on, half the phones in your 3560 will be up and the other half will take a minute or two before which they start powering up.

Attached is a log.

In the "sh power inline" output I have some of the ports from port 6 through 19 missing, but if you look at the syslog you can see that there are phones attached to those ports.

HTH

PS: Please remember to rate helpful replies!

New Member

Re: 3560 with 48 PoE phones

Thankyou all for your input to this thread.

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