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I have ethernet cabling throughout my home, using cat 6 ethernet cables underfloor to various points on the walls. In my office there are two points side by side, and I have connected a separate D-link DGS-1008D switch to each of them, again via cat 6 cables. Here's the strangeness:

  • When switch A is connected to point A via cable A, the switch shows green (gigabit).
  • When switch B is connected to point B via cable B, the switch shows yellow (100Mbit).
  • When switch A is connected to point A via cable B, the switch shows green.
  • When switch B is connected to point B via cable A, the switch shows green.
  • When switch A is connected to point B via cable A, the switch shows green.

Etcetera.

Trying every single combination proves that each switch, cable, and wall point is capable of gigabit throughput. It is just one specific combination of switch, cable and wall point that is incapable of it.

Although I have everything connected up now so that I am getting good performance, I'm somewhat bemused by the consistent failure of one particular combination. It wasn't just an intermittent problem, every time I try that arrangement again it reverts to 100Mbit. Does anyone have a feasible explanation for this?

Edit: here's a schematic of the layout

Fibre to the home
    ||
Chorus ONT box
    ||
Edgerouter Lite
    ||
Netgear GS108v4
    ||
D-Link DGS-1008D
 ||||||||||||||
(7 cables to a patch panel on the wall)
  ||                ||
   underfloor cabling
  ||                ||
(wall point with two RJ45 ports)
 port A            port B
  ||                ||
(cable A)         (cable B)
  ||                ||
D-Link DGS1008D   D-Link DGS1008D
 (switch A)       (switch B)

All cables are cat 6 UTP TGS568A, and the longest stretch is the underfloor cabling which is no more than 8m from the patch panel to the wall points. The other cables are all between 0.5m and 1.5m in length.

This photo is rather messy but the main feature is the 7 identical cables heading to the patch panel: From the ONT to the patch panel

And this is the double wall point (labelled 4 and 5 although in this question I have described them as A and B): switches A and B

In this arrangement, both switches A and B display a gigabit connection. If the two cables inserted in ports 4 and 5 are swapped over, then switch A reports gigabit, but switch B drops to 100Mbit.

I think it's clear that the problem must be the cable and the answer given earlier today probably explains why. I can't justify buying a $2000 cable tester to prove it though.

  • It may help to add a diagram of: cabling, wall-plates, apx cable lengths and visual examples of the "points". As it is the question is a little hard to interpret/visualize how this is all connected. Please add the additional info to the question and not the comments section. – Tim_Stewart Feb 25 at 19:25
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Every time you terminate a cat 6 cable you peel back the insulation, cut off the plastic separator core, unwind the wires a little bit, and mate a contact to the wire. Each of these adds a very small error to the RF/AC properties of the cable. A cable length limit of 100 meters also applies. It could be that one of your terminations isn't perfect, but just good enough to get gigabit in certain situations. Perhaps when you add up errors in the exact failing combination, the link refuses to negotiate gigabit.

If you plug a computer in, does it take about 15-30 seconds for the failing configuration to start passing data? This could be a sign the link is trying to negotiate gigabit because it sees continuity (DC) on the blue and brown pairs, but fails once the Ethernet signal (AC@125Mhz) signal is attempted. The DC and RF/AC properties of the cable are different, and the AC properties are very sensitive. This is also the reason a simple cable tester might indicate the cable is good. You can purchase a significantly more expensive cable tester that will tell you exactly how good or bad the cable is in terms of decibels of signal loss. It is much cheaper to just reterminate everything and try again.

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