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 AMPICCS. 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 errors outputs the error counters, where the FCS-Errors are indicating cabling problems. FCS stands for Field Checksum Sequence.

    #show 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 interfaces counters errors

Show power inline

The command show power inline shows the Power-over-Ethernet 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
Show power inline

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 favorite commands.

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

    #show 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
Test and show cable-diagnostics tdr interface

Show interfaces transceiver detail

Another one of my favorite 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
Show interfaces transceiver detail

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


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