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I was recently installing some servers, and making the cables the right length so that everything in the rack was tidy.

2 out of quite a few links failed to operate at gigabit, degrading to 100mb.

All cables had been tested, the thing that these 2 cables had in common, was that they were 60cm long, shorter or longer ones worked fine. Thinking about it, 60cm is the wavelength of a 125MHz gigabit signal in copper, so the 60cm cable may be something of a resonator.

Are there specific cable lengths that we are supposed to shy away from using?

  • There is no minimum length in the gigabit ethernet standards but did your testing show that your custom cables meet all the specifications set forth in the 1000BASE-T standard? Cheap cable testers only show that the pins are connected correctly, more expensive testers provide a more comprehensive test report. – HBruijn Aug 18 '16 at 15:40
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    Personally, I don't see the value (monetary or otherwise) in custom making your own cable unless you're having them made by a company that does that professionally and has the proper testing equipment and can certify the cables and provide you with the certification results. Manufactured, commercially available cables are made in almost any length imaginable and certainly can be found in lengths that meet or closely meet your length needs. – joeqwerty Aug 18 '16 at 16:14
  • You have not provided the rating of the cable...for Gigabyte I'd be using CAT 6 if you are not now doing that. – mdpc Aug 20 '16 at 0:21
  • @mdpc the cable was cat5e. The strange thing is I've made so many of these cables, of assorted lengths, longer and shorter, including many from the same reel of wire, and every single one has been fine, except for these 60cm ones. I checked the signal pairing to wire pairing. – camelccc Aug 20 '16 at 13:18
  • @camelccc And you made sure that each signal pair is assigned to a wire pair? – David Schwartz Aug 22 '16 at 11:01
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1000Base-T has no minimum cable length requirement.

60cm Gigabit cables are widely available. There's no particular reason a cable of that length would cause a problem. I suspect that your mapping of pins to wire pairs is incorrect. Most cable testers only confirm that each pin is wired to the corresponding pin on the other side. They don't check whether signal pairs map to wire pairs which is required for the cable to work reliably.

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Interesting question (and the resonance idea reminds me of the tesla coil I built many years ago).

Same cable length may suggest that the two devices are similar to one another. Is it possible that a device or switch port setting is hard-set to 100mpbs? (swap in a 'working' gigabit cable and see if the link speed increases).

Other ideas:

10BASE-T and 100BASE-TX use two pairs (4 wires of the 8 available) and 1000BASE-T/1000BASE-TX use three pairs (6 wires of the 8 available). Are you positive that you tested the cables for three corresponding pairs?

I believe CAT5 wire spec calls for a minimum bend radius of about an inch; if these cables are shorter, perhaps they are making some tight corners, in which case interference is more likely.

  • Any other cable, including other cables made from the same wire reel enables these ports to operate at gigbit speed. All 4 pairs are connected in both cases. The bend radius is an interesting idea, but constrained by the distance to the back of the rack – camelccc Aug 23 '16 at 12:35
  • One way to test your resonance theory would be to get a female / female rj45 coupler (Home Depot has these for about $5) and connect the questionable cable to a working gigabit cable and then try. The length of the copper in that case would not be a multiple of the resonant frequency of the signal, in theory. – Tony Aug 24 '16 at 15:52
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Gigabit Ethernet work on cables with any length, witch is less then 100 meters. There are two ways to spot the reason of your trouble. 1. Two cables are bad and you must renew them. Because Ethernet 100 Mbit/s works on two pairs and Ethernet 1000 Mbit/s works only on 4 pairs. 2. There is a network problem. It may be port speed settings, link aggregation, STP or something else.

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