SG300 PoE-short counters

[Helmeczi Zoltan]

Hi All,

We have ip cameras on a SG300-28PP switch, and one camera sometimes stop to working for a random amount of time (1/2day-2day).

After, something happens and this camera starts and working fine for a while. On the corresponding switch port i see short counters in PoE statistics.

SW3#sh power inline gi3

  Port      Powered Device         State          Status    Priority   Class

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

  gi3                               Auto            On      critical  class3

Time range:

Power limit: 30.0  W

Inrush Test: Enable

Port Status:               Port is on - valid capacitor/resistor detected

Overload Counter:          0

Short Counter:             52 

Denied Counter:            0

Absent Counter:            0

Invalid Signature Counter: 0

SW3#

Can you give me detailed explanation about short counters?

e.g.: What can cause this counter incrementation? or it is a switch related problem or can cause the cabling or the connected device?

Thanks,

Zoltan

[Helmeczi Zoltan]

Well, I will be sure to test this setting too.

And on next week we replace the faulty camera with a new one, and i will test with the faulty one and an another sg300 in lab environment.

And debug the negotiation phase with a C2960 for example to see what the camera does, why it ask class3 or why switch give class3 for this device.

I will post the results if i find something intresting or maybe the causes and solution.

Till then, thanks you for your useful help.

(Another question: Where should i ask manual or information about the SG300 debug-mode?)

[Dan Lukes]

As far as I know, there's no debug-mode documentation available to public. Only Cisco know, there's a documentation of such mode at all ...

[Dan Lukes]

Short counters monitors the number of power shortages encountered on the powered ports.

Glenn thank you for the response. But please allow me to rephrase your response to verify I understand it in full. I'm in doubt what events are considered "power shortage".

If a power request from a newly connected port can't be satisfied because of overall power budged exhausted, a power may be removed from already powered less critical device (if there's any) to satisfy the request from more critical device. The
short counter" is incremented in such case.

True ?

Is it possible the power will be removed from device with the same priority ? E.g. from critical device because of request from other critical device ?

[Glenn Martin]

Hi Dan,

True, you pretty much got it. It could be, however, you typically won't have all ports priority levels set to "CRITICAL", so it would compensate on the Low level ports. 

[Dan Lukes]

With no doubt. But it may happen, thus the question is still here:

Is it possible the power will be removed from device with the same priority ? E.g. from critical device because of request from other critical device ?

[Dan Lukes]

.