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show diagnostics tdr



Display the results of a time domain reflectometry (TDR) diagnostic test run on an interface. A TDR test characterizes and locates faults on twisted-pair Ethernet cables. For example, it can detect a broken twisted pair and provide the approximate distance to the break. It can also detect polarity swaps, pair swaps, and excessive skew.

The TDR test is supported on the following switches and interfaces:

  • EX2200, EX3200, EX3300, and EX4200 switches—RJ-45 network interfaces. The TDR test is not supported on management interfaces and SFP interfaces.

  • EX6200 and EX8200 switches— RJ-45 interfaces on line cards.

Use the request diagnostics tdr command to request a TDR test on a specified interface. Use the show diagnostic tdr command to display the last TDR test results for a specified interface or the last TDR test results for all network interfaces on the switch that support the TDR test.



Show summarized last results for all interfaces on the switch that support the TDR test.

interface interface-name

(Optional) Show detailed last results for the specified interface or a range of interfaces. Specify a range of interfaces by entering the beginning and ending interface in the range, separated by a dash—for example, ge-0/0/15-ge-0/0/20.

Required Privilege Level


Output Fields

Table 1 lists the output fields for the show diagnostics tdr command. Output fields are listed in the approximate order in which they appear.

Table 1: show diagnostics tdr Output Fields

Field Name

Field Description

Interface name or


Name of interface for which TDR test results are being reported.

Test status

Status of TDR test:

  • Aborted—Test was terminated by operator before it was complete.

  • Failed—Test was not completed successfully.

  • Interface interface-name not found—Specified interface does not exist.

  • Not Started—No TDR test results are available for the interface.

  • Passed—Test completed successfully. The cable, however, might still have a fault—see the Cable status field for information on the cable.

  • Started—Test is currently running and not yet complete.

  • VCT not supported on interface-name—TDR test is not supported on the interface.

Link status

Operating status of link: UP or Down.

MDI pair

Twisted pair for which test results are being reported, identified by pin numbers. (Displayed only when the interface option is used.)

Cable status

When detailed information is displayed, status for a twisted pair:

  • Failed—TDR test failed on the cable pair.

  • Impedance Mismatch—Impedance on the twisted pair is not correct. Possible reasons for an impedance mismatch include:

    • The twisted pair is not connected properly.

    • The twisted pair is damaged.

    • The connector is faulty.

  • Normal—No cable fault detected for the twisted pair.

  • Open—Lack of continuity between the pins at each end of the twisted-pair.

  • Short on Pair-n—A short-circuit was detected on the twisted pair.

When summary information for all interfaces is displayed, status for the cable as a whole:

  • Fault—A fault was detected on one or more of the twisted-pairs.

  • OK—No fault was detected on any of the twisted pairs.

Distance fault or

Max distance fault

Distance to the fault in whole meters. If there is no fault, this value is 0.

When summary information for all interfaces is displayed, this value is the distance to the most distant fault if there is more than one twisted pair with a fault.

Polarity swap

Indicates the polarity status of the twisted pair:

  • Normal—Polarity is normal. Each conductor in the twisted pair has been connected the same pins at the both ends of the connection. For example, a conductor connected to pin 1 at the near end of the connection is connected to pin 1 at the far end.

  • Reversed—Polarity has been reversed. For the twisted pair, the conductors have switched which pins they are connected to at the near and far ends of the connection. For example, the conductor connected to pin 1 at the near end is connected to pin 2 at the far end.

(Not available on EX8200 switches.) (Displayed only when the interface option is used)

Skew time

Difference in nanoseconds between the propagation delay on this twisted pair and the twisted pair with the shortest propagation delay. (Not available on EX8200 switches.) (Displayed only when the interface option is used.)

Channel Pair

Number of the 10/100BASE-T transmit/receive pair being reported on.

Pair Swap

Indicates whether or not the twisted pairs are swapped:

  • MDI—The pairs are not swapped (straight-through cable).

  • MDIX—The pairs are swapped (cross-over cable).

(Displayed only when the interface option is used.)


Indicates whether the connection speed is being downshifted:

  • No Downshift—No downshifting of connection speed.

  • Downshift occurs—Connection speed is downshifted to 10 or 100 Mbs. This occurs if the cable is a two-pair cable rather than the four-pair cable required by Gigabit Ethernet.

(Displayed only when the interface option is used.)

Sample Output

show diagnostics tdr interface ge-0/0/19 (Normal Cable)

show diagnostics tdr interface ge-2/0/2 (Faulty Cable)

show diagnostics tdr (All Supported Interfaces)

Release Information

Command introduced in Junos OS Release 9.0.