Leading Cisco networking products distributor-3network.com How to use time domain reflectometer (TDR)

1. How to use Time-Domain Reflectometer (TDR)
I’m no electrical engineer, (the closest experience I have with electricity is the amount of
electrocution I received when I was a child [due to faulty cabling of electrical appliances])
so I will spare readers technical jargons and boring formulas because this guide is not
aimed to be published in the International Journal of Mumbo-Jumbo. This guide is to
help anyone how to confidently use the TDR feature when troubleshooting basic
Layer 1 Ethernet issue.
My knowledge with this feature is based entirely on experience and a lot of trial-and-error.
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 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:
1

2. 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
Sup7E/X4548 Yes
Note:
2

3. 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
3

4. 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
3Command 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
4

5. 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”);
1Cable length is approximately 3 metres long and an error (length-wise) of 1 metre; and
2 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 down.
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
5

6. 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
6

7. 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.
Weird things have happened before
I’ve taken the opportunity to do limited testing on TDR on a 4510R+E. The chassis has a
Sup7E and with a X4548-RJ45V+ line card (IOS version 03.01.01.SG). The result(s) are
very, very weird. Oh, by the way, the TDR testing on this setup takes 60 seconds. 60
seconds! Good grief! I have no idea whether the Sup7E or the line card is the factor.
The sample below is coming from a GOOD cable:
Interface
Speed
Local pair
Pair length
Remote pair
Pair status
Gi2/36
1Gbps
1-2
29 +/-10m
Unknown
Fault
3-6
30 +/-10m
Unknown
Fault
4-5
29 +/-10m
Unknown
Fault
7-8
30 +/-10m
Unknown
Fault
7

8. And the sample below is coming from a BAD cable:
Interface
Speed
Local pair
Pair length
Remote pair
Pair status
Gi2/37
0Mbps
1-2
56 +/-10m
Unknown
Fault
3-6
0m
Unknown
Fault
4-5
56 +/-10m
Unknown
Fault
7-8
59 +/-10m
Unknown
Fault
As you can see, whether or not you have a good or a bad cable the result from the
“Remote pair” and “Status” can be deceiving. The ONLY WAY to determine if you have a
bad cable issue or not is to look at the “Speed” and the output to the “Pair length”.
I am suspecting that the misleading result of the “Remote pair” and the “Pair status” is an
IOS bug.
More related:
How To Recover Cisco Router Password
The Difference of The Cisco Catalyst 2900 and Cisco Catalyst 1900
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