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I have a machine that has a problem I've never seen and cannot understand.

The machine is a Dell laptop with Windows 7 Ultimate. The user tried to do something fancy with this machine (bridging the WLan interface with the Ethernet one in order to connect his Mac to his home network. Anyway...) and after he disabled his changes, the network stopped working properly on that system.

What happens is that the NIC can get a DHCP lease 1005 of the time but then packets will be randomly dropped. It will work for 10 seconds, then fail for 30 seconds, then work again, etc. The problem is the same whether we try to ping a local machine or the gateway.

What we've tried:

  • Booting from a Ubuntu live CD.
  • Clearing the ARP cache on the machine and switch.
  • Changing the NIC configuration to 100Mb half-duplex and disabled all offloading.
  • Changing the IP address.
  • Moving the machine to a different switch.
  • Connecting directly on the head switch.
  • Reinstalling the network card.

The only thing that worked was take the machine to a completely different network (an ethernet port on the WRT54GL wifi router that we use to connect customer to a dedicated ADSL line that has nothing to do with our internal network).

I'm out of ideas here. There is something that changed in our internal configuration and not the whole network is having a fit when it is asked to handle that mac address.

When I start a PING on the faulty machine and uses Wireshark on the destination, I see the same thing happening: when the ping on the machine fails, i see nothing in my trace. When the ping succeeds, I see the echo request coming in and the replies going out. The MAC address I see in the packets matches.

I'm really out of ideas here about what is happening. Anyone cares to make a suggestion ?

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  • Booting from a Ubuntu live CD. - and what happened? Are you saying it still failed when booted from Ubuntu?
    – Rob Moir
    Aug 4, 2011 at 13:38
  • Yep. Exactly the same thing happened: ping would randomly fail or succeed.
    – Stephane
    Aug 4, 2011 at 13:54
  • Have you tried replacing the network card?
    – Alan
    Aug 4, 2011 at 14:02
  • No. It's hard on a laptop.
    – Stephane
    Aug 4, 2011 at 14:03
  • So is it the MAC address or the network card? Find another machine that allows you to use a configurable MAC address and configure it to have the same MAC address as the suspect machine and wait to see if the symptoms follow the MAC address. Shut down first 'problem' machine while this is going on, of course, and make a note of the 'real' MAC address on the 2nd machine so you can fix it afterwards!
    – Rob Moir
    Aug 4, 2011 at 14:11

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It might be helpful to see if you can isolate the fault to the MAC address or the network card? Find another machine that allows you to use a configurable MAC address and configure it to have the same MAC address as the suspect machine and wait to see if the symptoms follow the MAC address. Shut down first 'problem' machine while this is going on, of course, and make a note of the 'real' MAC address on the 2nd machine so you can fix it afterwards!

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