Upgrade to gigabit switch made long cable stop working

We have two buildings connected by a 30m network cable with a switch on either end. Last week we upgraded some old equipment to support gigabit speeds. One of the two switches was an old 10/100 switch so we upgraded it. Right away the connection between the buildings stopped working.

What we've tried

1. Plug the old 10/100 switch back in in-between the long run and new gigabit switch. This works, but obviously destroys the upgrade.

2. The cable was a Cat5e that was installed 4 years ago. It's within the spec, but we thought it might be low quality / damaged, so we ran a brand new Cat7 cable. Same exact behavior as the old Cat5e (ie, it works on the 10/100 switch but not the gigabit switch).

3. We tried using different gigabit switches on both sides of the cable. Same behavior.

4. We plugged one end of the cable directly into a gigabit ethernet computer and the other end into the new switch. This worked fine, leading us to suspect there might be an Auto-MDI negotiation problem. We rewired the new Cat7 to be a crossover cable to avoid the need for Auto-MDI. Same behavior.

5. We connected both of the new switches using a short (2m) patch and cross cables. The connection works perfectly with either one.

What other troubleshooting options do we have at this point?

Equipment details:

• Old 10/100 switch - TP-Link TL-SF1005D
• New gigabit switch - TP-Link TL-SG1008D (produced in 2018)
• Old gigabit switch in building 2 - TP-Link TL-SG1008D (produced in 2016)
• Replacement gigabit switches we used to test - DLink DGS-1008C and DLink DGS-1008A

Solution 1:

Gigabit requires all 4 pairs, and 10/100 only requires 2 pairs. So "testing" it with the 10/100 switches doesn't really verify that all pairs of the cable are good.

For something like this, you will want to use a cable certifier, not a plain tester that just tests continuity.

30m really isn't that long of a run. If they are newer switches that support low powered Ethernet standards turn off Green Ethernet/EEE/whatever to see if this helps. If for whatever reason they are detecting the run as shorter than it is they might not be pushing enough power on the ports.

Is there any high voltage power running through the same conduit? Power and data should be at minimum 2" apart to avoid EMI (electromagnetic interference).

Solution 2:

It seems likely that the (consumer grade) switch devices are not suitable for the job in hand. You should use better switches - it's the only way to be sure.