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SYS64738
Asked: 2020-05-10 11:34:38 +0800 CST

Wrong link speed; what happens if manually set speed using ethtool fails?

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I've got a remote xen server and I noticed that the speed link in autoneg is 100 Full Duplex; all my network is 1000 Full Duplex; do you know any reason for this? Ethtool mentions on both my server advertised speed and link partner advertised speed (my switch), as well as other slower speeds, also 1000 Full Duplex. I was tempted to try to force the speed manually in this way:

ethtool -s eth0 speed 1000 duplex full autoneg off

Another command I could try, but I'm not sure if it could be useful, is:

ethtool -s eth0 speed 1000 duplex full autoneg on

Obviously my concern is that the link will break and this is a remote server; I can access it (it's just 2 miles away) but I'd prefer to avoid it (home working);

I would like to just test the manual speed and reverse to autoneg if it fails, I'm thinking to use crontab to set autoneg on every hour, but I don't find it a good strategy.

What would you do in this case?

performance xenserver ethtool

1 Answers

  1. Best Answer

    If autonegotiation is resulting in 100Mbps, it's highly probable that:

    • the other side is configured to 100Mbps (or is 100Mbps hardware).
    • there's a fault on the physical layer (i.e. the cable has degraded or a connector isn't making contact properly).

    Assuming that you know that neither side is 100Mbps hardware and you haven't reconfigured the NIC or switch on the other end to be 100Mbps in configuration, you can assume that it's the cable or connection.

    If you want to get a bit more of a definitive answer remotely, you could possibly try a copper test if possible. Even though the link is taken down, it's automatically re-enabled when the test is complete, usually after about 30-60s. It would be a more reliable, professional approach than a jury-rigged cron job; it would also potentially yield information on precisely what's wrong with the cable and where the fault has occurred. However note this wouldn't actually yield any fix, as most likely it would indicate you need to go to the site to fix it.

    If you however force 1000Mbps full duplex then the link will most likely go down. Assuming a physical issue, autonegotiation is negotiating a slower speed because not all of the data lines are connected. If you force 1000 F/D, the NIC needs all 4 pairs in order for the link to come up. Not only would you not learn anything, but you are causing yourself risk and more work.

    In answer to your question about what I would do, I would wait until I was visiting the site and make sure I have a replacement patch cable with me. In such a situation if you leave anything to chance you can assume that you're just going to end up cutting yourself off, causing you to feel like a berk and giving yourself a "fire to fight" in the process.

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