What is Time-Domain Reflectometer (TDR) - Have your switch test your cable!

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:

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?

• Switches need to run IOS version 12.2 or later. TDR is supported in IOS version 15.0. IOS version 12.0 and 12.1 do NOT support TDR.

• If you are running IOS version 12.2(46)SE or earlier, TDR test is DISRUPTIVE. During the test, the interface will go down and up. For obvious reasons, anything connected will lose network connectivity.

• If the remote-end device is a power-over-ethernet (PoE) device, the test will cause the device to lose power. If you have, for example, a Voice over IP (VoIP) phone and a PC client is connected to the phone, both the phone and client will lose network connectivity because the phone does not have a bypass functionality. This will affect ALL IOS versions.

• Particularly when you are running old IOS versions, the test can take between five (5) to seven (7) seconds.

• TDR works on 10/100/1000BaseTx. Fibre optic ports (any flavours) is not covered/discussed here. TenGigabitEthernet copper port DOES NOT (YET) support TDR.

• Cisco GLC-T/GLC-TX SFP module does NOT support TDR.

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

• 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. Pairs C and D will report a result of “Not Supported”.

• 2. Except the WS-C2960-48PDL, when using the copper GigabitEthernet (Gig 0/1 and Gig 0/2) ports of the Catalyst 2960, one must manually set the interface to copper using the command “media rj” before the test can be conducted.

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?

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