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I have a LAN that has 4 Macs and a router / switch to connect them all.

My Ethernet LAN speed on my iMac is pathetically slow. I additionally have a MacMini, MacBook and G4 in my LAN, they all go way faster. For measuring I used a 220GB file to copy from one machine to another. While between the MacMini and MacBook this estimates to take about an hour, when copying to my iMac this measures at about 15 hours.

I then unplugged the cable from the iMac and plugged it into the MacBook to check if it might be a cable, and the speed was fine. So the cables seem to be fine.

I then connected the MacBook directly to my iMac bypassing the router and the speed was fine too, so the iMac port seems to be fine.

So, I’m thinking it’s something in the settings. On the iMac, I have two partitions, one running 10.11.6 and one running 10.6.8. On both systems I got the same results. When looking at network Utility, it shows Gigabit.

I reset the router and switch, restarted the iMac and am out of ideas right now.

  • It can't possibly be the iMac settings. You're iMac has no idea it's connected to a switch or directly to the MacBook it just sends/receive packets through a socket attached to the network device. If it works properly when you remove the switch, the issue is the switch. The one thing I noticed you didn't try was different ports on the switch or resetting the switch to clear it's tables. – Allan Mar 10 '18 at 17:14
  • thanks Allan, but what I did is, I connected a different Mac on the same cable and then it worked flawlessly. That's why I ruled out the switch, but still I resetted it to no avail. So, it works properly when I remove the switch, and it works properly when I change the machine. – Busty Mar 10 '18 at 17:47
  • The MAC (media access control) table in the switch is probably corrupt. Try another one (a cheap one is fine). – Allan Mar 10 '18 at 18:00
  • edit: post was obsolete – Busty Mar 10 '18 at 18:01
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    Hey Allan, that was a good one! My router has four ports, so I just connected all the machines directly to it and voila, speed is good. So, the switch - or it's table as you point out seems to have been the problem.Thanks very much, you helped me a lot! – Busty Mar 10 '18 at 18:09
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This problem has the feel of a data packet size mismatch causing fragmentation with corresponding delay. Possibly combined with the machine hardware for some reason being set up for half-duplex communication. For more information about packet size you can use the ping command from terminal with different packet sizes as described in this article: https://community.hpe.com/t5/Networking/What-detail-symptoms-will-I-be-getting-if-MTU-size-mismatch/td-p/6909407 Compare the ping results between the fast and the slow computers on the same physical connection.

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As Allan pointed out, the switch was the problem. Without the switch the speed is as expected for a gigabit network.

All credit & thanks to Allan.

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