SG250-08 negotiate too much PoE power (CPD) from PSE

[etihaenni]

Edit: Change title as it's look like the problem come from the SG250-08 PD. See next post.

Hello,

I would like to report/share a bug I discover on the SG250-08HP switch. It run the last actual firmware (2.4.5.71) and it supply power to a SG250-08 switch.

The switch report a consumed PoE power of 31W, but the real used power is only 5.2W.

Best regards

[etihaenni]

In fact, it's look like the problem come from the SG250-08 PD device. During boot of the PD device, the SG250-08HP (PSE) display a power consumption of 5W, and once it's up, the indicated power is 30W. In the datasheet, the power consumption of the SG250-08 is mentionned to be 7.6W. This means that the SG250-08 negociate too much power (30W instead of 7.6W) to the PSE. It can be seen in the syslog event:

I check other PDs (Cisco SG200-08, Cisco WAP121, Intellinet 561082) and the consumed power is correctly displayed.

Cisco, please correct this bug. I hope this bug won't prevent other PD's to get their needed power with the actual firmware.

Kind regards.

[etihaenni]

The problem is effectively in the SG250-08 device when it is a PD.

In the Discovery-CDP, Local information we can see that the SG250-08 does negotiate 30W, but it real power consumption is less than 10W according to datasheet, and the temperature of this fanless device :) 30W would made it untouchable.

Is there a way to correct this without a firmware correction?

On the firmware side, it's look quite easy to correct, it may just be a constant value to modify :)

Kind regards.

[mipopov]

Hello Etihaenni,

What happens if you disable CDP and leave only LLDP running?

[etihaenni]

Hello,

Thank for your interest to this issue.

As you propose, I disable CDP on the PD switch and reboot it, which is confirmed by the fact that the PsE switch does no more find a CDP neighbor, but the reserved power is still 30W. So disabling CDP does not resolve the issue.

Kind regards.

[mipopov]

Hello Etihaenni,

Can you enable SSH on the switch and attach the output from the following commands:

*note - enable logging in external file because the output is usually more than the scrollback line count

switch# show tech

switch# show mac address-table interface gi1

switch# show power inline 

switch# show power inline interface gi1

Thank you in advance

[etihaenni]

Hello,

Can you please tell me how to enable logging to external file?

By the way, only the following command lines does work, will it be sufficient for you?

switch#: show tech

switch#: show mac address-table

switch#: show power

Kind regards

[mipopov]

Hello, 

For the external file logging:

*in Windows:

Download Putty -  link below:

http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

Set Putty to save everything in a file:

- Session > Logging -- there, change None to Printable Output

- and choose folder with Browse

[etihaenni]

Ah yes, how can I forget PuTTy. Please find attached the output log of the PD switch with CDP disabled.

Kind regards

[etihaenni]

For information, I reboot the 08HP (PSE) switch this evening and now the negociated power through LLDP is 25.5W. I hadn't see a message in the log of the PSE switch that it will keep the CDP reservation until the gi1 port is cycled down/up.

Reserving 25W for the SG250-08 switch looks quite excessive nowing that the switch is provided with a 12V/1A (12W) power adapter. Would it be possible to correct this power negociation in the next firmware release?

Or the bug is as initialy suspected in the 08HP (PSE) switch which shows the "reserved power" in the "consumed power" field instead of the real consumed power.

Kind regards

[etihaenni]

Hello,

Any update about this issue?

I install the last firmware for these switches and the behavior is still the same.

Kind regards

[peeterp]

Hello,

There may be a work-around by in that it appears to be possible to force the SG250-08 PD to negotiate the 802.11AF mode,

resulting in reported consumed power at power source (such as SG250-08HP) to be 15W only.

Kind regards