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I encounter that I can reach 1000mbps with some combinations of RJ45 outlets and cables, and only 100mbps with other combinations.

  • I can reach 1000mbps with all cables (when combined with a given RJ45 outlet)
  • I can reach 1000mbps with all outlets (when combined with a given cable)
  • I can reach 1000mbps with cables that are longer than the ones I can only reach 100mbps with

So I conclude this is not an issue of

  • switch
  • PC nic
  • connection switch <-> RJ-45
  • cables alone
  • RJ45 outlet alone
  • cable lengths

but rather an issue of the combination of some.

Configuration

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Observations

Cable Outlet Speed
Cable 1 RJ-45 1 1000mbps
Cable 2 RJ-45 1 1000mbps
Cable 1 RJ-45 2 1000mbps
Cable 2 RJ-45 2 100mbps

This is 100% reproducible.

Question

How can it be that cable 2 + outlet 2 only give 100mbps, while they are individually both capable of getting to 1000mbps?

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2
  • Have you have switched the ends of each cable (thus reversing them) in each of the above tests?
    – Astara
    Commented Dec 28, 2020 at 21:38
  • What are the types of cable? What are the types of Outlet? Are they all the same? Are they hand-made or prefab? How long are the cables? (There are many details that affect signal on the line. I've seen long CAT5e cables that drop to a lower speed at long distances due to signal degradation. Anything that reduces signal quality will do this.)
    – HackSlash
    Commented Dec 28, 2020 at 21:59

3 Answers 3

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The simple solution would be to replace the outlet with a CAT6 RJ45 outlet receptacle. If there is a weak connection in the port, combined with some flaw in the cable, that results in signal degradation then the act of putting on a new outlet will give you a more solid connection. This could make up for the weakness in the bad cable but I would replace that too if it's just a patch cable.

Or get a Fluke Ethernet cable tester and test every part of the system if you really want to know why.

This one shows distance to fault so it gives you an approximation for where the flaw is:

https://www.fluke.com/en-us/product/network-cable-testers/copper/ciq-100

When plugged in to a faulty port you should see a flaw very near the meter.

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How about this?

  • one (or more) of your cables is crossover cable
  • some of your network devices can't handle that? (not supported or disabled auto MDI-X)
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  • Except auto MDI-X is baked into the gigabit standard :<
    – davidgo
    Commented Dec 28, 2020 at 22:50
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Possible issue:

The cable #2 is if lower/poor quality. Outlet #2 either has longer cable than #1 or has been terminated poorly.
When you combine those together, the signal drops enough for network cards to fall back to lower speed.
It does not happen with cable #1 and Outlet #1, because they are better quality and compensate for the shortcomings of the weaker part enough to keep 1Gbit speed.

How to fix: Reterminate cable 2 (if hand made). If you don't have tools or it's a store bought cable with molded jacks, junk the cable and get new one of respectable quality.
Also check and optionally re-terminate the outlet #2. Often just punching the wires in again does the trick.

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