Testing cables from a Cisco IOS-XE Cat 9300 Switch and How to use Time-Domain Reflectometer (TDR)
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).”
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:
- Local-end Side (LeS) patch cable;
- Local-end Side (LeS) patch panel (including punch block);
- Horizontal cable;
- Remote-end (Red) patch panel (including punch block);
- Remote-end (Red) patch cable; and
- Remote-end (Red) device NIC
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:
- Command to start the TDR: “test cable tdr interface <interface of your choice>”;
- Wait for about 5 to 7 seconds for the test to run; and
- Command to show the result of the TDR test: “show cable tdr interface <interface of your choice>”
Now let’s see what the I results would look like.
So what does this result above tell us?
- Port tested is on GigabitEthernet 0/1;
- Port has negotiated to 1 Gbps;
- Cable use is a straight-through cable (look and compare the values of “Local pair” and “remote pair”);
- Cable length is approximately 3 metres long and an error (length-wise) of 1 metre; and
- All four pairs are working fine (Pair status)
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 |
Lets Looks some real example from Cat 9300 :
Running the TDR Test
To start the TDR test, perform this task:
Procedure
| Command or Action | Purpose | |
|---|---|---|
| Step 1 | test cable-diagnostics tdr {interface { interface-number }} | Starts the TDR test. |
| Step 2 | show cable-diagnostics tdr {interface interface-number} | Displays the TDR test counter information. |
test cable-diagnostics tdr interface Tw1/0/27
show cable-diagnostics tdr interface Tw1/0/27
show cable-diagnostics tdr interface Tw1/0/1
show cable-diagnostics tdr interface Gi4/0/26
When there is no device connected :
Happy Labbingggggggggggg!