Checking Faulty Cables

I recently had to work with a 3rd part to diagnose a link between our devices and came across this handy command. The link in question was a pretty hefty (75m-ish) UTP cable run between a Cisco and HP switch. I have visibility of the Cisco switch, into the structured cabling into the patch panel, and the 3rd parties cable. Unfortunately I didn’t have a DC Operations tech with access to a Fluke, or the ability to interpret the output of a Fluke, but they did have a laptop with a 100Mbps NIC (this becomes important later on).

So I started by running the diagnostic on the production connection. It’s not working, so I don’t have to worry about taking stuff down. This gives me the following:

test cable-diagnostics tdr interface gi7/21
TDR test started on interface Gi7/21
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.

switchA#show cable-diagnostics tdr interface gi7/21

TDR test last run on: July 09 10:30:20
Interface Speed Pair Cable length Distance to fault Channel Pair status
——— —– —- ——————- ——————- ——- ————
Gi7/21 auto 1-2 77 +/- 6 m N/A Invalid Terminated
3-6 75 +/- 6 m N/A Invalid Terminated
4-5 75 +/- 6 m N/A Invalid Terminated
7-8 N/A 8 +/- 6 m Invalid Open

It doesn’t really tell me anything, apart from that there’s most likely, an infrastructure fault. This could be a faulty cable, patch panel, or a faulty switchport.

I now start my troubleshooting. I need to ascertain the path I have visibility of. The tech tells me there is structured cabling to the line card on the switch, so I get him to connect his laptop to the section the B end connects to, and I run the diagnostic.

switchA#show cable-diagnostics tdr interface gi7/21

TDR test last run on: July 09 10:31:55
Interface Speed Pair Cable length Distance to fault Channel Pair status
——— —– —- ——————- ——————- ——- ————
Gi7/21 100 1-2 16 +/- 6 m N/A Pair A Terminated
3-6 16 +/- 6 m N/A Pair B Terminated
4-5 N/A 12 +/- 6 m Invalid Short
7-8 N/A 11 +/- 6 m Invalid Short

Interestingly, this works, but at 100Mbps. I also see that a couple of pairs in the cable are not terminated correctly. Pair D is for 1Gbps and pair C is for PoE. This was turned up by a quick Google search. I also see that the fault is located +/-6m from the switchport, suggesting the problem is at my end.

I double check this against a working port:
Another_switch#show cable-diagnostics tdr interface gigabitEthernet 7/16

TDR test last run on: July 09 10:18:43
Interface Speed Pair Cable length Distance to fault Channel Pair status
--------- ----- ---- ------------------- ------------------- ------- ------------
Gi7/16 1000 1-2 0 +/- 6 m N/A Pair B Terminated
3-6 0 +/- 6 m N/A Pair A Terminated
4-5 309 +/- 6 m N/A Pair D Terminated
7-8 99 +/- 6 m N/A Pair C Terminated

I now reconnected the customer end, and set the port speed to 100Mbps manually using

interface Gi7/16
speed 100

Unsurprisingly, the port comes up. The cable diagnostic shows the same output as it did above. I suspect the problem is either a faulty punch down on pair D, an incorrectly crimped cable, or the customer manually setting their speed to 100Mbps for some reason.

Now usually, if this were structured cabling to a new switch, I would ask for the patch panel terminations to be punched down again. However, this time, I know the cable is the newest part in the puzzle and ask the tech to re-crimp it. It works, and is now up at 1Gbps.

I now have learnt something new about cabling pairs. There are a few things to be aware of.
– The “Distance to fault” field can throw up false positives.
– I do not know how a correctly terminated cable connected to a 100Mbps will output. Will it see the cable D termination even if it is not being used for communication?
– The connection drops momentarily when the diagnostic is run. Remember to be aware of this if troubleshooting a working connection.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s