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user3280964
Asked: 2021-03-26 07:32:28 +0800 CST

Gigabit RJ45 loopback not working (100 Mbps ok)

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I made myself a loop back plug as per

RJ45 loopback

  • When I plug it into a 1Gbps port on 100/1G switch the port does not light up.
  • 100 Mbps port on 100 Mbps switch lights up no problem.

The obvious problem could be that I crimped the cable wrong, but now I'm on the 3rd one and I get exactly same frustrating results. I triple checked my pinout and I just can't get it to work. I'm not new to crimping cables either, so I'm 99% sure I succeeded in crimping it well. I also tried several switches, and it's always the same: 100Mbps lights up, 1Gbps doesn't.

The other weird thing is that 1Gbps switch that's 100Mbps capable doesn't detect this at least as 100Mbps (I could live with it not showing up as 1Gig). But it stays completely dark. But maybe that's to be expected because that's also what happens when I plug in my other 2 pair 100Mbps RJ45 loopback ... so maybe 100Mbps works ONLY on 100Mbps switch? I could live with that if that's how that works, but what about 1Gbps loopback with 4 pairs? Why is that not lighting up?)

Can anyone shed some light on this? I just don't get why it's behaving the way it is. Am I doing this right?

loopback rj45

1 Answers

  1. Best Answer

    Turns out "Gigabit loopback is a limited concept"

    Gigabit NICs have crosstalk detection (detects how much signal interferes onto other wires), and will likely decide that the loopback is an extreme amount of crosstalk - any may not show link. Meaning it’s often only useful on NICs which let you disable crosstalk detection. Gigabit switches may behave differently (but I’m not sure what the spec says or the real-world variation is)

    source

    Also, even commercial loopback adapters come with a lengthy caveat regarding gigabit compatibility

    not all gigabit Ethernet systems support loopback operation. The Gigabit Loopback Jack & Plug is intedned solely for testing systems where the Near End Crosstalk (NEXT) function can be disabled and the equipment under test can support being looped back to itself. Consult the equipment manual of the equipment being tested to determine if gigabit loopback is supported

    source

    From all my testing and research on this subject my conclusion is that unless you're dealing with exclusively ancient 100Mbps equipment, the loopback adapter concept is now obsolete due to advanced signalling in modern switches. For a diagnostic tool like this to be useful, it has to work 100% of the time, but this experience has shown that it's very flaky.

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