Comcast gigabit modem, ethernet question

After an extended comedy of errors (where do they GET those support people?) I now have a functioning Comcast gigabit cable modem. WiFi is giving me great speeds, especially compared to the 100Mb/s service I had until today.

For 5-6 years I have been using an ethernet cable, rated for direct burial, in conduit, to provide WiFi and ethernet connection in the shop. The run is about 250'
In the shop it plugs into a Tp-link gigabit switch and a cable from there goes to the shop computer.

Something doesn't make sense to me.

There are four ethernet ports on the the modem numbered 1,2,3,4 with a orange stripe painted beside #4.
Comcast tech support said it didn't matter which port I used.
I first plugged in the ethernet cable to the shop into port #1. The Ookla speed test gave me about 60Mb/s in the shop, even worse than with the old 100 Mb/s modem.
Comcast tech support said it was because my ethernet cable was too long. I didn't think so.

I tested the modem ethernet port #1 by connecting a laptop with a 4' cable. I got nearly 1000 Mb/s.

I then plugged the cable to the shop into modem port #2 instead of #1. Now I get nearly 1000 Mb/s in the shop!

For anyone still with me, is there any logical technical reason for the modem ethernet port #1 to be moving packets at only 6% of the rate on port #2?

The only thing I can imagine is a physical thing. Perhaps some wire in my ethernet cable plug is not making good connection when plugged into port #1, but twisting the connector 180 degrees to plug it into port #2 causes it to make a good connection. This doesn't give me a good feeling! Maybe I should cut off the plug and crimp on a new one. (I wired that cable six years ago and neither end has strain relief.)

BTW, I did connect a cheap Tp-link mesh node to the same gigabit switch in the shop to test the concept and it works well. I'm getting over 500Mb/s over the WiFi, more than I need for my iPad and phone!

JKJ

When I upgraded to gigabit I found that a number of the Cat5 cables and connectors I had were not capable of the speed. A poor wire connection, or an unshielded bit of wire in the wrong place can kill your speed. I discovered that the morons who wired my house thought they were working with POTS cables and stripped the shielding back 6-8" to make them easier to work with. OK at low speeds, but not for fast connections. Replacing short wires with factory Cat8 cables and re-terminating the ones with the unshielded runs with Cat8 shielded connectors solved all of my problems, except for the wire that feeds my primary WAP on the main floor of the house-- they didn't leave enough cable in the wall to pull it out even three inches, much less clip it off and re-terminate. that one gives me 2-300 Mb/s, re-running that cable will be a real PITA project one of these days. The Cat5 cables seem to support the speed just fine, it's the terminations that are killers. The Cat8 terminators seem very robust.

Is that first port on the Comcast device (which is a router, too) nailed up to something slow in configuration, like 100mb and half-duplex? Something is amiss with the port and that would be the very first thing I would check.

Quote:

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Originally Posted by Jim Becker

Is that first port on the Comcast device (which is a router, too) nailed up to something slow in configuration, like 100mb and half-duplex? Something is amiss with the port and that would be the very first thing I would check.

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The port was fine (full speed) when tested with a laptop and a short ethernet cable.

Quote:

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Originally Posted by John K Jordan

The port was fine (full speed) when tested with a laptop and a short ethernet cable.

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Ah, you did mention that. Sorry. I guess that port just doesn't like your cable...