How to troubleshoot an ethernet drop

A former Macworld editor found himself baffled at an ethernet problem in a new house.

ethernet cable

Lex Friedman used to edit and write at Macworld—a lot, lot, lot of articles—and now works at a leading podcast, ad-sales, and content network. He called me after moving into a new house recently as he thought he had lost his marbles somewhere in the wiring of his house.

Lex’s problem was that the house was pre-wired for ethernet, which is generally a good thing. He got a new broadband modem from his ISP and purchased a new ethernet switch, but when he went to plug in his AirPort Base Station (a pre-2012 model), it wouldn’t work in one place on the network, but would on another.

  • When plugged via his new switch into the broadband modem in his basement, the base station worked fine, getting an Internet connection without a problem.
  • When connected via one of the house’s internal ethernet “drops”—the run in the walls from jack to jack—to the basement switch, the base station couldn’t obtain an IP address.
  • However, when he plugged the base station into the modem in the basement, and plugged in the switch in upstairs, the switch had Internet access over ethernet.

Something was afoot. ethernet cables can go bad, and because it’s relatively rare, most people don’t have to troubleshoot this regularly. It seemed like either one of the ethernet patch cables or one of the runs in the walls of the house was funky, but his testing didn’t show a consistent problem because the network worked in some configurations and not others.

What was even more maddening is that the LED status lights on the switch and the base station both lit up green, and the light blinked on the switch showing activity. If the cable were truly inoperable, neither light should be lit up; some switches will show a different LED color or pattern for a bad cable, too.

I generally recommend isolating causes, including marking and swapping ethernet patch cables in case one is in bad shape, and moving the cable around can cause intermittent problems—sometimes it works, but twisted a different way, it doesn’t. (Also, never wrap your ethernet cables in a bow or tightly. Roll them into a neat circle, letting them roll as they wish instead of twisted to fit.)

Lex went back through and isolated all the internal runs and found an oddball situation: one of the connections in his house, even when nothing was plugged in at the end of it, would cause the internal network for fail. It likely has an electrical short, which isn’t dangerous—ethernet is both low voltage and low amperage, and would only carry a charge when connected to a switch or device at other end. Otherwise, it’s inert.

The solution is to run a new cable, tying the new one to the old and pulling it carefully into place, assuming the conduit is wide enough to pass a new cable. Internal wiring like this should always used plenum-rated cabling, which is rated for fire resistance and low smoke and off-gassing.

To make his network work, he just left that one run disconnected, and will eventually replace it with a new wire.

Postscript: After years of proper operation of an ethernet switch in my basement, connecting base stations in my house, it failed a couple of hours after I helped Lex troubleshoot his problem. This just lends credence to the theory that hardware talks to each other.

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