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Problems with a Passive Network Tap on Gigabit PoE


Skinny
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Has anyone used a passive network tap (i.e. throwing star tap or diy) to capture traffic while connected to a gigabit PoE switch? I'm targeting a VoIP phone and am getting spotty results. Here are the details:

  • Phone: Grandstream GXP2130
  • Switch: Netgear GS108PE
  • Tap: Similar schematic to the throwing star tap

The phone boots just fine using PoE with the tap in line and negotiates a stable 10/100 Mbps connection as expected. When plugging into the receive side of the tap, the phone drops the network connection momentarily but recovers. It's inconsistent but sometimes I can capture a small amount of traffic in Wireshark. On the transmit side, I get absolutely nothing. 

If I disconnect the phone from the PoE ports and plug it into the regular gigabit ports, I have more success. Both transmit and receive can be captured, but the phone has be powered from a normal wall power outlet for this to occur. 

I'm curious if anyone else has had the same experience? I would really like to be able to capture traffic while the phone is plugged into the PoE port.

Also, if you have a PoE switch that is not gigabit, do you have similar issues? 

Thanks for any help at all!

-Skinny

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Solved it! Had to build out the tap circuit a little more to take care of the PoE power. I think the phone and switch was having a bit of a loading issue with the capture laptop being on the line. I'll be conducting some stability tests to make sure this solution sticks.

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I'm not sure I follow what you mean by daisy chaining a second non-PoE switch. 

The laptop isn't taking any power from the PoE switch, but it is most definitely causing an imbalance on the line. The connection from switch to phone it extremely fragile in this case. If I disturb the line at all by plugging into the Tx or Rx side of the passive tap, the phone momentarily drops off the network. When stability returns to the line and the phone recovers, the capturing laptop has problems and will not see the traffic or will only see it sporadically.

The new design places some blocking capacitors on the tapped conductors in an effort to keep any inadvertent DC draw occurring due to the presence of the laptop. So far this solution has worked.There are a few more cases I'd like to test to make sure the solution is as robust as I hope.

To replicate the problem I'm seeing, try to capture traffic using the Throwing Star LAN tap with a gigabit VoIP phone connected to a gigabit PoE switch.

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1 hour ago, Skinny said:

I'm not sure I follow what you mean by daisy chaining a second non-PoE switch. 

I mean plug a non-PoE switch into the first switch, and then plug the laptop into the second switch (daisy-chaining is just linking lots of the same thing, basically).

You could still see if it does anything - only if you've got another switch lying around. The aim is to reduce the immediate load on the first switch and see if it helps. If so, there's probably an issue with that switch.

It may not help, though.

Edited by Dave-ee Jones
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