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Cool Tool for Solving Layer 1 Cable Issues


Csaba Farkas, Ygomi, Debrecen, Hungary.

The Catalyst 2960, 2970, 3560/3560-E, and 3750/3750-E switches have an integrated Time Domain Reflector (TDR), which is used to test cables associated with a port. TDR is supported only on 10/100/1000 and some 10/100 (Catalyst 2960) copper Ethernet ports. It is not supported on 10 GigabitEthernet or SFP module ports.

Switch# test cable-diagnostics tdr interface gigabitethernet0/2
TDR test started on interface Gi0/2

A TDR test can take a few seconds to run on an interface. Use "show cable-diagnostics tdr" to read the TDR results.

Switch#show cable-diagnostics tdr interface gigabitEthernet 0/2
TDR test last run on: Dec 10 09:05:10

Interface    Speed    Local pair    Pair length    Remote pair    Pair status
---------    ------   ----------    -----------    -----------    -----------
Gi0/2          auto     Pair A        22 +/- 4 m         N/A       Open
                        Pair B        21 +/- 4 m         N/A       Open
                        Pair C        5 +/- 4 m          N/A       Open
                        Pair D        20 +/- 4 m         N/A       Open

The above example output shows the results of a TDR test, which was run while the other end of the cable was disconnected(Open). Because the other end of the cable is open, the "Remote pair" mapping is N/A (not available). From the results, we can see that Pair C has a "Pair length" that is much shorter than pair A, B, and D. This suggests there is an open (break) in pair C at the 5 meter point (+/- 4 meters) before the end of the cable (approximately 20 meters +/- 4 meters) is reached.

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