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A remote office has a Cisco ASA 5505 connecting the LAN to our HQ LAN via VPN.

When a particular Windows XP PC is connected to the ASA via a switch, the VPN tunnel drops, and after a short while, all connections through the ASA stop working. Disconnect the PC and it all starts working again.

The only time I have seen anything like this happen was when someone created a network loop by connecting two network ports on a switch together, where spanning-tree was disabled. However, we have traced all the cables in this small remote office, and it does appear to be a single PC/NIC causing the problem.

Is there any configuration that could be put on the PC that might result in this sort of behaviour? If so, I can clean it up. If not I can clearly assume that the NIC is faulty, and replace it.

BTW - I am assuming this is a simple layer 2/3 network related issue affecting the router or switch functionality of the ASA. It could of course be something else. Cables and basic network configuration (IP address, Link Speed/Duplex) all checked. ClamWinPortable scan from a USB drive found nothing. No sign of unauthorised software.

Any other suggestions would be welcome.

I would just get a new NIC if this PC wasn't in rural Sierra Leone... If only someone could invent a farcaster or teleportation device.

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Does the computer have the same IP as the ASA? – jwbensley Feb 3 '12 at 19:08
No - it's set to get it's IP address from the ASA (of course, I didn't check to see whether it was getting an IP address from the ASA - isolated from the network as soon as we narrowed it down.) – dunxd Feb 4 '12 at 18:16
Run wireshark/tcpdump on the offending coputer perhaps, and see what it is sending/receiving. – jwbensley Feb 5 '12 at 12:53

Can I offer what may seem like a silly idea? Check the type of cable they're using. Also, check the cabling on that switch(unless it's in common use by others already). I'm saying all this because it sounds like a cable issue. Have the user try another cable too and another switch(if possible).

share|improve this answer
Yeah - we swapped the cable out for a known good one, and checked the existing cable on another computer. Not a cable issue... – dunxd Feb 4 '12 at 18:13

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