Latitude E7440 Ethernet port problem

I have a really weird issue with the Ethernet port on my E7440.

Cable A is 25 feet long. If I connect Cable A directly to the laptop, it will show that the cable is unplugged. If I connect Cable A to the docking station that the laptop is connected to, it will show that the cable is unplugged.

If I connect Cable A to a switch, then from a switch, I use a shorter 6 ft cable to connect to the laptop or the docking station, then it would work.

I have also switched out the E-port replicator with a few other replicators, and it would show the same issue. I've switched out the Cat5E cable, and anytime I use a cable that is longer than 6ft, it would not work.

However, if I connect Cable A directly to another Latitude E7440, then it would work just fine.

Any idea of this is a faulty Ethernet port or something else?