Yesterday I finished running about 40 meters of FTP Cat6 cable through the electrical conduit, from the first floor to the attic. Both ends of the cable are terminated as T568B on Cat6 wall jacks. So, I now have a Cat6 ethernet jack in my living room and another Cat6 jack in my studio in the attic.

To test that everything was OK, I connected a cat5e cable from the living room jack to the main router, and then up in the attic I connected another cat5e cable from the ethernet jack to a laptop. And absolutely nothing happened, the laptop didn't even recognize a cable was connected. A bit puzzled, I then used a network cable tester to see if the cable was indeed OK, and, to my astonishment, the tester said it was a crossover cable.

I double checked the termination on both ends and they are indeed T568B. The rest of my network and all my cables are T568B as well. It then occurred to me that maybe using Cat5e cables to complete the connection from my router to the laptop was the problem, as I am effectively mixing Cat5e and Cat6 cables in a single connection, if you know what I mean. Could that be the problem? I won't be able to test the connection with Cat6 cables until later this afternoon, but wanted to know whether mixing Cat5e and Cat6 cables in the manner I've done can cause the connection to be labelled as crossover when, in fact, it should be straight.

  • It cannot be a problem of coupling cat5 and cat6 cable. Never. One of the cable (most probably the one you did) have to be crossed. Check color by color... – Hastur Sep 14 '16 at 20:23

No. On of the pieces of the connection (either Cat5 cable or the Cat6 cable) is a cross over. In addition make sure you're not using an uplink port on your router as that might be crossed. Up to date routers and switches usually should be able to use MDI-X to automatically switch whenever it should be a crossover port or not.

Test both Cat5 cables whenever they're straight or not. If both are straight chances are that the Cat6 is a crossover, maybe because someone wasn't paying attention while assembling the termination points.

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  • Both Cat5e cables are straight, as I've checked them with the tester. The Cat6 cable that runs through the wall should also be straight. I terminated the cable myself and have double checked that the pins are arranged as T568B at both ends. – madelarra Sep 14 '16 at 8:03
  • Does your tester show the connection for those three cables as straight? If not one of the cables just isn't straight. If it does either of your endpoints has a problem. Just think about what actually happens when you connect the jacks to each other. It's essentially just connecting the wires to each other. If each is straight the whole cable (consisting of three parts) should be straight. – Seth Sep 14 '16 at 8:08
  • Exactly, it should be straight! The only cable I can't test by itself is the one on the wall, at least not with my tester, as the jacks on the wall are female (obviously). The only thing I can think of is that maybe the stickers on the female connectors indicating the pin arrangements for both 568B and 568A are wrong. But, what are the odds? – madelarra Sep 14 '16 at 8:16
  • If you know both Cat 5 cables are straight you can just plug them in as an extension to get male connectors. You know they are straight. If you're able to, you could as well just have a "top down" view on the connection to/in the socket and compare it to the one on the other end. If they're not the same (color coding) they're not straight. So both should have color x on pin 1, color y on pin 2 etc. – Seth Sep 14 '16 at 8:52
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    I'll do some further testing this afternoon when I get home from work. I will update this post accordingly. Thanks. – madelarra Sep 14 '16 at 9:29

Strange as it may sound, the problem was with the stickers that came attached to the female connectors. They came the wrong way round, so to have a T568B ternination I had to place my cables as T568A.

So, the problem had nothing to do with mixing Cat5e and Cat6 cables, but rather with some pretty poor QA procedures.

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  • I'm not sure if you can but don't forget to mark it as the solution, if it's possible. :) – Seth Sep 15 '16 at 5:26

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