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Switch: How to test a cable status?

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Anand G
Cisco Employee

Works on 29xx, 37xx, 45xx & 65xx series model switches

Checking the Cable Status Using the TDR

You can check the status of copper cables using the time domain reflectometer (TDR). The TDR detects a cable fault by sending a signal through the cable and reading the signal that is reflected back to it. All or part of the signal can be reflected back by any number of cable defects or by the end of the cable itself.

Use the TDR to determine if the cabling is at fault if you cannot establish a link. This test is especially important when replacing an existing router, upgrading to Gigabit Ethernet, or installing new cables.

The port must be up before running the TDR test. If the port is down, you cannot enter the test cable-diagnostics tdr command successfully, and the following message is displayed:

router#test cable-diagnos tdr int gig0/1

% Interface Gi0/1 is administratively down

% Use 'no shutdown' to enable interface before TDR test start.

Note: TDR can test cables up to a maximum length of 115 meters.

How to run the test?

router # test cable-diagnos tdr int gig0/1

TDR test started on interface Gi0/1

A TDR test can take a few seconds to run on an interface

To Check the Result:

'show cable-diagnostics tdr int gig0/1' will provide the TDR results.

A Sample Run Result:

switch#sh ip int bri | i up

GigabitEthernet0/1     unassigned      YES unset  up                    up     

switch#

switch#test cable-diagnostics tdr interface gigabitEthernet 0/1

TDR test started on interface Gi0/1

A TDR test can take a few seconds to run on an interface

Use 'show cable-diagnostics tdr' to read the TDR results.

switch#

*Mar  1 01:59:05.231: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to down

*Mar  1 01:59:06.238: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to down

*Mar  1 01:59:12.772: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up

*Mar  1 01:59:13.779: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

switch#

Output:


switch#show cable-diagnostics tdr interface gigabitEthernet 0/1

TDR test last run on: March 01 01:59:04

Interface Speed Local pair Pair length        Remote pair Pair status

--------- ----- ---------- ------------------ ----------- --------------------

Gi0/1     1000M Pair A     8    +/- 10 meters Pair B      Normal             

                        Pair B     8    +/- 10 meters Pair A      Normal             

                        Pair C     8    +/- 10 meters Pair D      Normal             

                        Pair D     8    +/- 10 meters Pair C      Normal             

switch#

Regards,

Anand G

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