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Cable Troubleshooting Example

Another job and another example of an Ethernet coupler causing challenges.

In this case, we upgraded some old 10/100Mb switches to 1 Gb switches. As part of my methodology, I typically get all the devices connected in, make sure everything is up and running, then clear the port statistics (when possible), check the ports for speed/duplex and errors after a minimum of an hour or so of operation.

You can do this via CLI, web interface, customized scripts or SNMP. As long as you can reliably collect the data, you are in good shape. Don’t forget to document how you gathered the data, date, time, and collection period.

In this example, I noticed the connection between the switches was running at 100Mb full duplex which was odd since both switches have 1 Gb ports. I grabbed my cable tester, but as you will see in the video, this isn’t a requirement. Tested from the tester directly connected to the switch ports and both tested at 1 Gb full duplex. Then I used the existing switch-to-switch connection with the cable tester and it came back as 100 Mb full duplex. Ran a TDR and saw that there was a spike midway through the cable, indicating the cable has been repaired or reconnected in some way. I went for a walk and Bingo!, an inline Ethernet coupler. Looked pretty old and no one knew it was there because "everything worked fine".

I didn’t have a coupler to swap it out with, so I used an unmanageable gig switch since they had a few laying around as a test point. I disconnected the coupler and connected the cables to the switch. The 1 Gb port lights on the switch lit up and I confirmed on both switch ports that the corresponding ports were now 1 Gb. We cleared the port counters, let the traffic run through the port as we cleaned up and labeled equipment. We came back an hour later and confirmed that the port was running fine with no errors.

As I said in the video, I did not recommend they leave the switch in permanently, especially this one being a switch-to-switch port. I told them they basically have 2 options; replace this switch with a better manageable switch or pull a new cable run.


Tony Fortunato

Sr Network Performance Specialist

The Technology Firm

Getting things to work better - bit by bit-


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