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A custom ethernet cable with both connectors wired as T568B only works when connecting A to B, but not reversed, i.e. when switching ends and connecting B to A.

A is a Mac, B is a 1Gb TP-Link unmanaged switch. Tried with different Macs and switches. “Works” means both devices see each other (the LED in the switch's ethernet port lights up, and on the Mac's System Preferences > Network, “ethernet” has a green dot) and the Mac can connect to the rest of the network.

What might the reason be?

closed as off-topic by Zac67, Jenny D, Ward Feb 23 at 1:56

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  • 1
    What do you mean with "custom" ethernet cable? It's probably due to bad wiring. – Tommiie Feb 14 at 9:41
  • With “custom” I mean I cut the cable to the appropriate size and wired the connectors to both ends myself. With “bad wiring” you mean the wire might be bad, that the connectors might not be properly wired as T568B (improbable), or that some cables might not be properly crimped? Wouldn't any of the above prevent A to B from working too? – elmimmo Feb 14 at 9:43
  • Is the cable working in one direction at a speed of 1Gbps or a speed of 100Mbps? – Tommiie Feb 14 at 9:52
  • 1Gbps according to /System/Library/CoreServices/Applications/Network Utility.app > Info > Link Speed – elmimmo Feb 14 at 11:33
  • Get a cable tester. – Jenny D Feb 21 at 17:19
3

Put simply, you've made a bad cable. It could be any of the reasons you list, use a cable tester to verify it. Trying to work out why/how it's bad is mostly pointless since the solution will be to cut the connectors off and try again if it doesn't make the cable too short.

For the sake of having to select a length rounded to the nearest whole metre, just buy a pre-made Ethernet cable and avoid the hassle, you can even pick from a range of colours.

  • Used a cable tester. It confirms continuity is OK. – elmimmo Feb 15 at 13:04
  • At any rate, after testing, I did cut one of the connectors off and tried again, and now works. But I am intrigued about why the tester would report continuity and the cable would consistently work when connecting devices in one direction but not the other. – elmimmo Feb 15 at 13:30
  • Maybe one of the pins wasn't quite crimped properly and the Ethernet socket on one device pushed the pin in a little more than on the other end. Who knows? – bodgit Feb 15 at 13:54

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