I have network topology:

      (room 3)
      D-Link DES-1024D
      (room 2 MAIN)                              (room 4)
      Baseline 3COM  Switch 2948-SFP Plus ------ D-Link DES-1024D
      (room 1)         
      D-Link DES DES-1016D

Problem is in room 3 D-link switch. There are about 15 computers connected to that switch, all works fine but suddenly and unpredictably link goes down. It may happen once in a month or every 15 minutes.

Room 3 totaly goes offline. Сomputers can not even see each other. The rest of the network works fine. Cable reconnect only can bring room back online.

3COM's switch has a monitoring system. It showed many CRC & Align Errors and Fragments on the port which is room 3 connected.

I tried to connect room 3 to room 4. Now all traffic from the room 3 goes through the room 4 switch which is not critical but gave me the opportunity to see now all CRC & Align Errors and Fragments (all of which is bad packets, i guess) goes through room 4 port on 3COM switch.

How can I determine where these bad packets was generated? Can it be ethernet card? Or just buy new switch?

1 Answer 1


It can't be PC ethernet card, as DES-1024D is not 'cut-through', that means, if PC sent broken frame, DES-1024D dropped it and did not forward it.

You've already done good debugging by moving the wire to another switch, this allows us in high degree of confidence to exclude 3COM as culprit for the errors.

Options that are left is room 3 D-Link or wiring from it. First thing I'd verify is duplex setting, if negotiation is on (as it should be, only disable if you must, as negotiation has important function to signal remote fault) verify that both of the devices are in 'full duplex', if auto negotiation is on but links still disagree on duplex force them to full duplex.

If this does not help, you have three options

  1. Change port on the room 3D-Link. Fault may affect one, many or all ports in the device, depending on which exact component is broken and how many ports share this component
  2. Rewire (if room 2 and room 4 to room 3 are not sharing any wire, you can skip this)
  3. Change room 3 D-link
  • thank for answer. i think i'll start with changing port on the room 3 and then will buy another switch if problem will continue to appear.
    – antongrt
    Jul 3, 2013 at 9:48

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