Friday, November 20, 2009

Troubleshooting Layer 1 Cabling Issues - Cisco Catalyst Switches TDR Feature

Cisco TDR (Time Domain Reflectometer)

In Cisco IOS Release 12.1(19)EA1 or later, you can use the Time Domain Reflector (TDR) feature to diagnose and resolve cabling problems. When running TDR, a local device sends a signal through a cable and compares the reflected signal to the initial signal.

TDR is supported only on 10/100/100 copper Ethernet ports. It is not supported on 10/100 ports, 10-Gigabit module ports, or on SFP module ports.

TDR can detect these cabling problems: 

Open, broken, or cut twisted-pair wires—The wires are not connected to the wires from the remote device.

Shorted twisted-pair wires—The wires are touching each other or the wires from the remote device. For example, a shorted twisted pair can occur if one wire of the twisted pair is soldered to the other wire.

If one of the twisted-pair wires is open, TDR can find the length at which the wire is open.

Use TDR to diagnose and resolve cabling problems in these situations:

Replacing a switch

Setting up a wiring closet

Troubleshooting a connection between two devices when a link cannot be established or when it is not operating properly 


core-switch#test cable-diagnostics tdr interface gi0/24
TDR test started on interface Gi0/24
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.

core-switch#show cable-diagnostics tdr interface gigabitEthernet 0/24
TDR test last run on: November 19 14:54:18

Interface Speed Local pair Pair length        Remote pair Pair status
--------     -----   ----------- --------------       -------------- ---------------
Gi0/24    1000M  Pair A     4    +/- 4  meters Pair A        Normal             
                        Pair B     4    +/- 4  meters Pair B        Normal             
                        Pair C     5    +/- 4  meters Pair D        Normal             
                        Pair D     4    +/- 4  meters Pair C        Normal

It means Gi0/24 is running on 1000 Mbps speed, All four pairs have cable length of 4 meters & corresponding remote pair side (for local pair) is working Normally.

The previous command is set to be deprecated sometime in the future and will be replaced by:

" show diagnostic result interface GigabitEthernet 0/24 "

Deepak Arora

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