What is Time-Domain Reflectometer (TDR) - Have your switch test your cable!
Wednesday, November 16, 2011 at 10:41AM
George

This is a handy trick to test your cable from a Cisco switch. My buddy Leo wrote this up.

 

 

What is Time-Domain Reflectometer (TDR)?

“A time-domain reflectometer (TDR) is an electronic instrument used to characterize and locate faults in metallic cables (for example, twisted wire pairs, coaxial cables)1.”

 

For the sake of this document, “TDR testing” and “TDR” are used interchangeably in this document to sow confusion to the un-initiated. They both mean the same.

How can TDR help me?

TDR, in its simplest form, can help you determine IF you have a cable problem, WHICH pair(s) is/are faulty and HOW FAR away the fault is.

 

Typically, when you have a Layer 1 issue there are a lot of factors to consider:

  1. Local-end Side (LeS) patch cable;
  2. Local-end Side (LeS) patch panel (including punch block);
  3. Horizontal cable;
  4. Remote-end (Red) patch panel (including punch block);
  5. Remote-end (Red) patch cable; and
  6. Remote-end (Red) device NIC

 So you see, dear readers, TDR minimize the guess-work.

 

 

Picture this …

Before we begin, let me give you the “lay of the land”. Presume the following scenario:

 

Drawing1.jpg

 


What model of Cisco switch does TDR work on?

Firstly, not all switch model support TDR. TDR feature first came out with the Catalyst 2960. So here is the list of which ones will work and will not:

 

Model

TDR Support

2960

Yes1, 2

2960G

Yes

2960S

Yes

2918

Unknown

2350

Unknown

2360

Unknown

2975

Unknown

3560

No

3560G

Yes

3560E/3560X

Yes

3750

No

3750G

Yes

3750E/3750X

Yes

Nexus 2K

Unknown

Nexus 5K

Unknown

Nexus 7K

Yes3

 

 

Note:  

1.        The 2960 will support TDR in both the FastEthernet and dual-personality GigiabitEthernet port, however, when used on a FastEthernet port, TDR will only test the first two pairs, namely Pairs A & B.  For obvious reasons, Pairs C and D will not be tested when used on non-GigabitEthernet ports.

2.       Except the WS-C2960-48PDL, when using the copper GigabitEthernet port of the Catalyst 2960, one must manually set the interface to copper using the command “media rj” before the test can be conducted.

3.       Confirmed by Cisco TAC, Ankur Garg.

 

The list does not include modules/blades for the Catalyst 4000/4500, 5000/5500, 6000/6500 although it is mentioned here that TDR was introduced with IOS Release 12.2 ZY for the Catalyst 6000/6500. It’s not included in the list above because I don’t have the resources to test and verify.

 

Legacy Cisco Catalyst models 1900, 2900XL/3500XL, 2940/2950/2955, 2948G and 2970 are not supported. Routers are also not supported. I do not have any resources to test router Ethernet Switch Modules (NME, HWIC, EHWIC). Wireless Access Points do not support TDR.

 

Why doesn’t the FastEthernet-flavoured 3560 and 3750 support TDR and but the cheaper FastEthernet 2960 support TDR?

 

Base on the time-line, the “plain” (or non-GigabitEthernet copper port) 3560 and 3750 came out BEFORE the 2960. The “chip” for the TDR was included in the design of the 2960. When Cisco released the 3560G and 3750G later, someone made the ultimate decision to include the TDR feature as a standard. Therefore, the plain 3560 and 3750 are the only two series that WON’T HAVE the TDR feature. (Take note reader: Emphasis on the words “WON’T HAVE”)

 


Any Gotchas I need to be aware of?

 

 

 

 

 

 

The next two Gotcha items are for those who plan to use the TDR feature on Cisco Catalyst 2960 and 2960G (2960S not included):

 

 

 


How to use TDR?

The commands are very simple: One to start the test and the second command to display the result. Here is simple procedure:

 

  1. Command to start the TDR: “test cable tdr interface <interface of your choice>”;
  2. Wait for about 5 to 7 seconds for the test to run; and
  3. Command to show the result of the TDR test: “show cable tdr interface <interface of your choice>”

 

See? Easy! Now let’s see what the I results would look like.

 

Interface

Speed

Local pair

Pair length

Remote pair

Pair status

Gi0/1

1000M

Pair A

3 +/- 1 meters

Pair A

Normal


Pair B

3 +/- 1 meters

Pair B

Normal


Pair C

3 +/- 1 meters

Pair C

Normal


Pair D

3 +/- 1 meters

Pair D

Normal

 

So what does this result above tell us?

 

  1. Port tested is on GigabitEthernet 0/1;
  2. Port has negotiated to 1 Gbps;
  3. Cable use is a straight-through cable (look and compare the values of “Local pair” and “remote pair”);
  4. Cable length is approximately 3 metres long and an error (length-wise) of 1 metre; and
  5. All four pairs are working fine (Pair status)

 

Under “Pair status” you can get the following results:

 

Result

Explaination

Normal

Ideal result you want.

  • If testing FastEthernet, you want Pair A and B as “Normal”.
  • If testing GigabitEthernet, you want ALL as “Normal”.

Open

Open circuit. This means that one (or more) pair has “no pin contact”.

Short

Short circuit.

Impedance Mismatched

Bad cable. For more explanation, go here.

 

An ideal result is “Normal”. In practice, whether the remote-end device is FastEthernet or GigabitEthernet, I will never accept a TDR result other than “Normal” in all four pairs.

 


Cable Pairs explained?

 

This is how I see what each Pairs control:

 

Pairs

Function

A

This pair controls whether or not the port should go up or not.

B

Protocol-level and controls FastEthernet.

C

Power over Ethernet (PoE)

D

GigabitEthernet

 

More examples

 

Interface

Speed

Local pair

Pair length

Remote pair

Pair status

Gi0/11

100M

Pair A

13 +/- 1 meters

Pair B

Normal


Pair B

12 +/- 1 meters

Pair A

Normal


Pair C

0 +/- 1 meters

Pair D

Open


Pair D

0 +/- 1 meters

Pair C

Open

 

Normally, this result would freak me out. Look at the items in RED. Pairs C and D are reporting a cable value of “0”. Next I move to the “Pair status” and it’s reported as an Open circuit. No pin contact. Whao! But look at the speed. It’s 100 Mbps. So it’s normal … I guess.

 

But wait. What if the remote-end side (Red) client is a GigabitEthernet. So where is the faulty cabling? Which one of the patch cables? Or is it a horizontal cabling? Does the client support GigabitEthernet or not?

 

Here’s another clue: Look at the length of the cable for Pair A and B. It’s reporting around 12 to 13 metres. Experience has taught me that my Local-end Side (LeS) cable doesn’t exceed two metres. So that rules out my cable, however the horizontal cabling is more than 10 metres. So what’s between the horizontal cabling and the remote-end client? You have three suspects: 1) The remote-end punch block; 2) the remote-end patch cable; and 3) remote-end client.

 

Culprit was the remote-end punch block and the horizontal cabling: Cable contractors only terminated two pairs.

 


Never ask a boy to do a man’s job!

 

Interface

Speed

Local pair

Pair length

Remote pair

Pair status

Gi1/0/48

auto

Pair A

149 +/- 1 meters

Pair B

Normal


Pair B

151 +/- 1 meters

Pair A

Normal


Pair C

35 +/- 1 meters

Pair D

Short/Impedance Mism


Pair D

21 +/- 1 meters

Pair C

Short/Impedance Mism

 

Its results like the ones above that makes me want to cry.

 

Ok, I look under “Pair status” and I see “Short/Impedance Mism” for Pair C and D. No question about it. It’s bad cabling. This is not what makes me want to cry. Look at under “Pair length” of Pair A and B. NOW cry.

 


Should I be worried?

 

Interface

Speed

Local pair

Pair length

Remote pair

Pair status

Fa0/39

100M

Pair A

6 +/- 1 meters

N/A

Open


Pair B

49 +/- 1 meters

N/A

Open


Pair C

N/A

N/A

Not Supported


Pair D

N/A

N/A

Not Supported

 

Looking at the result, I can confidently say that the appliance was a 48-port Cisco Catalyst 2960. How? Look under “Interface”. Look at “Pair status” for Pair C and D. Only the plain 2960 FastEthernet ports can support TDR.

 

But look at “Pair status” for Pairs A and B. What does that mean?

Drawing2.jpg

 

 

It means that the remote-end (Red) patch cable is missing.

Article originally appeared on my80211.com (http://www.my80211.com/).
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