I have a LAN set up with 6 or 7 PC's connected some wired and some wireless. Also there are a number of Arduinos connected with ethernet shields which do data collection.

I have been having problems with my DSL connection and have isolated it to one PC, a tower with an Asus board and AMD phenom processor running Debian Wheezy. This machine has been operating for a least 5 years. It is always on, it is the database server for the data collected by the Arduinos.

As soon as I plug its RJ45 connector to the LAN (to the router) my DSL modem quits working. The ADSL stays on but all Rx and Tx lights quit and non of the other computers can access the internet. The LAN continues to operate normally, ping times are normal, etc.

When I unplug the RJ45 of the offending computer my DSL connection goes back to normal.

This seems to have happened suddenly and nothing else has changed on the LAN. I'm pretty sure that there are no conflicting MAC or IP addresses. The main router is a Cisco / Linksys E4200. The other switches are mostly D-Links and there is one pair of Engenius WAPs set up as a bridge.

Any ideas about what could be wrong?

Thanks.

For the PC that is causing the problem, what happens if you add a new network card to it and use that new network card to connect that PC to your LAN?

Have you tried to use another port on switch?

Well, this is embarassing, my problem was an ip address conflict. I had three routers set up as WAP's that all had the same ip address. Two of them have been that way for over ten, yes 10, years. I thought (based on manufacturers documentation) that once you set a router to WAP only IP settings don't matter any more, but NO!

It is even made worse because I had a "somewhat" similar problem when I set two Arduinos up with the same IP.

Why it blew up all of a sudden, I don't know. Knock on wood.

P.S.: Emerson, thanks for the Master Foo link.