09 Oct

Troubleshooting Cisco PoE with built-in TDR features

I recently encountered some issue, no PoE given to the ip phone and saw below lines in the switch log:

%ILPOWER-3-CONTROLLER_PORT_ERR: Controller port error, Interface Gi0/16: Power given, but Power Controller does not report Power Good

SWITCH#sh power inline
Available:370.0(w) Used:0.0(w) Remaining:370.0(w)

Interface Admin Oper Power Device Class Max
--------- ------ ---------- ------- ------------------- ----- ----
Gi0/1 auto off 0.0 n/a n/a 15.4
Gi0/2 auto off 0.0 n/a n/a 15.4
Gi0/3 auto off 0.0 n/a n/a 15.4
Gi0/4 auto off 0.0 n/a n/a 15.4
Gi0/5 auto off 0.0 n/a n/a 15.4
Gi0/6 auto off 0.0 n/a n/a 15.4
Gi0/7 auto off 0.0 n/a n/a 15.4
Gi0/8 auto off 0.0 n/a n/a 15.4
Gi0/9 auto off 0.0 n/a n/a 15.4
Gi0/10 auto off 0.0 n/a n/a 15.4
Gi0/11 auto off 0.0 n/a n/a 15.4
Gi0/12 auto off 0.0 n/a n/a 15.4
Gi0/13 auto off 0.0 n/a n/a 15.4
Gi0/14 auto off 0.0 n/a n/a 15.4
Gi0/15 auto off 0.0 n/a n/a 15.4
Gi0/16 auto faulty 0.0 n/a n/a 15.4

Sometimes it can be an IOS bugs, some workaround is to reboot the switch. But before doing that it’s better if we use Time-Domain Reflectometer (TDR) features to diagnose the physical cabling.

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

So you see, TDR minimize the guess-work.

To start diagnostic:

SWITCH#test cable-diagnostics tdr interface gigabitEthernet 0/16

wait for the test to start, after 4-7 seconds, run below command

SWITCH#sh cable-diagnostics tdr interface g0/16

TDR test last run on: October 09 10:46:53

Interface Speed Local pair Pair length Remote pair Pair status
Gi0/16 auto Pair A 49+/- 4 meters Pair A Normal
Pair B 48 +/- 4 meters Pair B Normal
Pair C 49+/- 4 meters Pair C Normal
Pair D 49+/- 4 meters Pair D Normal

Based on above result, we know the cable having an issue. So what does this result above tell us?

Port tested is on GigabitEthernet 0/16;
Port has negotiated to auto;
Cable use is a straight-through cable (look and compare the values of “Local pair” and “remote pair”);
Cable length is approximately 49 metres long and an error (length-wise) of 4 metre; and
All four pairs are working fine (Pair status)

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

Result Explanation
Normal 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

Cable Pairs explained?

Pairs Functions
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

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.


Source: https://supportforums.cisco.com/docs/DOC-18983

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