Useful Cisco Commands for Troubleshooting cable problems

Problematic ICCS cabling

During a project where I was onsite I discovered a 20-25 years old network cabling, called AMP ICCS. This cabling system takes one Cat.5 cable, splits the eight wires into 2x four wires, which serve two ports in a network outlet.

This type of cabling is limited to 100 MBit / Full duplex and only capable for 15.4 Watts Power-over-Ethernet. 1 GBit or new wireless access points which require PoE+ with 30 Watts (like Cisco's 2800 Series) are not working with this type of cabling system.

ICCS Cabling System

ICCS Cabling System

If I encounter a "troublesome" network cabling, I'm always using the following commands on a Cisco switch for troubleshooting and hunting for Layer one problems. In my examples shown below, I'm troubleshooting interface Gi1/0/4 on my 3750G Switch:

Show interfaces counters errors

The command show interfaces counters errorsoutputs the error counters, where the FCS-Errors are indicating cabling problems. FCS stands for Field Checksum Sequence.


#sh interfaces counters errors

Port        Align-Err     FCS-Err    Xmit-Err     Rcv-Err  UnderSize  OutDiscards
Gi1/0/1             0           0           0           0          0            0
Gi1/0/2             0           0           0           0          0            0
Gi1/0/3             0           0           0           0          0            0
Gi1/0/4             0           0           0           0          0            0

Show power inline

The command show power inline shows the PoE usage, which is useful to check if a device like an IP phone or a wireless access point is getting the PoE power they need. In this output, it shows my Cisco IP Phone 7975G which consumes 12 Watt.


#show power inline

Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
1           370.0       58.2       311.8
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Gi1/0/1   auto   off        0.0     n/a                 n/a   15.4
Gi1/0/2   auto   off        0.0     n/a                 n/a   15.4
Gi1/0/3   auto   off        0.0     n/a                 n/a   15.4
Gi1/0/4   auto   on         12.0    IP Phone 7975       3     15.4

Test cable-diagnostics tdr interface

The command test cable-diagnostics tdr runs a cable diagnostics test, with the show cable-diagnostics tdr command the output can be viewed. The command shows the wire pairs and also the length. This is one of my favourite commands.


#test cable-diagnostics tdr interface gigabitEthernet 1/0/4

#sh cable-diagnostics tdr interface gigabitEthernet 1/0/4
TDR test last run on: February 21 21:45:25

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- -------------------
Gi1/0/4   1000M Pair A     21   +/- 4  meters Pair A      Normal
                Pair B     20   +/- 4  meters Pair B      Normal
                Pair C     20   +/- 4  meters Pair C      Normal
                Pair D     21   +/- 4  meters Pair D      Normal

Show interfaces transceiver detail

Another one of my favourite commands is show interfaces tranceiver detail, which provides a lot of information of the SFP or SPF+ modules inserted into a switch.


#sh interfaces transceiver detail module 1
mA: milliamperes, dBm: decibels (milliwatts), NA or N/A: not applicable.
++ : high alarm, +  : high warning, -  : low warning, -- : low alarm.
A2D readouts (if they differ), are reported in parentheses.
The threshold values are calibrated.

                              High Alarm  High Warn  Low Warn   Low Alarm
          Temperature         Threshold   Threshold  Threshold  Threshold
Port       (Celsius)          (Celsius)   (Celsius)  (Celsius)  (Celsius)
--------- ------------------  ----------  ---------  ---------  ---------
Gi1/0/25    32.1                90.0        85.0        -5.0      -10.0
Gi1/0/26    34.8                90.0        85.0        -5.0      -10.0
Gi1/0/27    34.8                90.0        85.0        -5.0      -10.0
Gi1/0/28    34.8                90.0        85.0        -5.0      -10.0

                              High Alarm  High Warn  Low Warn   Low Alarm
           Voltage            Threshold   Threshold  Threshold  Threshold
Port       (Volts)            (Volts)     (Volts)    (Volts)    (Volts)
---------  ---------------    ----------  ---------  ---------  ---------
Gi1/0/25   3.29                  3.60        3.50        3.10       3.00
Gi1/0/26   3.29                  3.60        3.50        3.10       3.00
Gi1/0/27   3.37                  3.60        3.50        3.10       3.00
Gi1/0/28   3.28                  3.60        3.50        3.10       3.00

           Optical            High Alarm  High Warn  Low Warn   Low Alarm
           Transmit Power     Threshold   Threshold  Threshold  Threshold
Port       (dBm)              (dBm)       (dBm)      (dBm)      (dBm)
---------  -----------------  ----------  ---------  ---------  ---------
Gi1/0/25    -5.0                 0.0        -3.0        -9.5      -13.5
Gi1/0/26    -5.0                 0.0        -3.0        -9.5      -13.5
Gi1/0/27    -5.0                 0.0        -3.0        -9.5      -13.5
Gi1/0/28    -5.0                 0.0        -3.0        -9.5      -13.5

           Optical            High Alarm  High Warn  Low Warn   Low Alarm
           Receive Power      Threshold   Threshold  Threshold  Threshold
Port       (dBm)              (dBm)       (dBm)      (dBm)      (dBm)
-------    -----------------  ----------  ---------  ---------  ---------
Gi1/0/25    -7.0                 3.0         0.0       -17.0      -21.0
Gi1/0/26    -5.6                 3.0         0.0       -17.0      -21.0
Gi1/0/27    -7.1                 3.0         0.0       -17.0      -21.0
Gi1/0/28    -4.6                 3.0         0.0       -17.0      -21.0

Note: Thanks to Heinz Ulm for finding a typo in the commands.

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