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.
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.