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?