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I recently replaced a 100Mbit switch with a gigabit one, and tested that two computers plugged directly into that switch could, indeed, share data at gigabit speeds (by the simple and obvious technique of stuffing a gigabyte of data down the wire and seeing how long it takes). All working beautifully. Fast-forward a few days, and I'm seeing that file transfers are slowish again; ethtool eth0 on one of the systems shows that it's dropped to 100Mb/s. ifdown eth0 and ifup eth0 gets network speed showing 1000Mb/s, and data began flowing at roughly 20MB/s.

What could cause the network interface to be dropped to a lower speed?

Debian Wheezy on the affected system.

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  • What type of cable? Cat 5?
    – Mark Plotnick
    Feb 8, 2015 at 15:37
  • Yep, Cat 5 (though usually I like to use 5e or 6).
    – rosuav
    Feb 9, 2015 at 16:45
  • Cat 5 ought to work (up to 100m), but Cat 5e or Cat 6 may work more reliably. If your cables are known to be good and there's no obvious interference (e.g. from the cables being near fluorescent light ballasts), you can try changing the autonegotiate setting on your switch and your computer to force gigabit and fullduplex.
    – Mark Plotnick
    Feb 9, 2015 at 16:59
  • Reviving this question with some more info. I just got myself a completely different switch, and a brand new cable that's clearly labelled 6E. It's also just half a meter long, and I'm confident there's no interference. The switch reports that the device on the other end is 100Mbit. Either I have a faulty NIC (and it's an on-board one), or I have something really weird happening.
    – rosuav
    Aug 14, 2015 at 7:04

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If it were me, I'd be thinking cabling problems. I might just have an aura that causes cabling to fail, but that's almost always what I find in this type of situation. Try new cables. Try different ports on your new gigabit switch. See if you can find a cable that acts differently between machine A and switch and between machine B and switch. Be thorough and be systematic.

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    The computer will keep error counters (as will the switch if it has web or ssh management) -- those counters should be zero. The output of dmesg should not show repeated connections and disconnections.
    – vk5tu
    Feb 9, 2015 at 6:14
  • If by "error counters" you mean the ones in ifconfig, then yes, they're staying zero.
    – rosuav
    Feb 9, 2015 at 16:32
  • In case it's faulty cable, I've just replaced it; both the old cable and the new one are clearly labelled Cat 5, fwiw.
    – rosuav
    Feb 9, 2015 at 16:46

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