Troubleshooting Interfaces

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Hello Syedwaqas.

When troubleshooting fibre based interfaces, there are some specific items that you should check:

First of all, all of the issues brought up by Rene in this lesson are applicable to fibre optic connections as well. Additionally, you should keep the following in mind when troubleshooting fibre based interfaces:

  1. Check the SFP or GBIC status by issuing the show inter status command. Here is an example from a production 3750 production switch that I have:
3750_DC_1#show inter status
Gi1/0/1   ***VOICE_SERVERS** connected    901        a-full a-1000 10/100/1000BaseTX
!-<output omitted>-!
Gi1/1/3                      notconnect   1            auto   auto Not Present
Gi1/1/4                      notconnect   1            auto   auto Not Present
Te1/1/1   ***LINK_TO_6506A_T connected    trunk        full    10G SFP-10GBase-LRM
Te1/1/2   ***LINK_TO_6506B_T connected    trunk        full    10G SFP-10GBase-LRM
3750_DC_1#

Notice that interface Gi1/1/3 which is an SFP slot says Not Present. This means there is no SFP module installed. Also, notice on the 10 Gigabit interfaces Te1/1/1 and Te1/1/2 it says 10G SFP-10Base-LRM. It tells you what SFP module is installed. If you have a module installed and it says Not Present then it’s quite likely that there is a problem with the SFP or the port.

  1. When troubleshooting fibre interfaces, keep in mind that specific types of SFP or GBIC modules are compatible with specific types of fibre optic cabling. Make sure you are using the correct patch cord and SFP module for the type of fibre optic cabling installed in your structured cabling.

  2. Keep in mind that even if you use the wrong kind of cable (multi mode instead of single mode or vice versa) you MAY get connectivity, albeit unstable connectivity. (Believe me, it has happened to me!) This is where you’ll need to check errors on the interface using the show interface command. For example, on the same production switch:

3750_DC_1#show inter te1/1/1
TenGigabitEthernet1/1/1 is up, line protocol is up (connected) 
  Hardware is Ten Gigabit Ethernet, address is 2c54.2d79.881d (bia 2c54.2d79.881d)
  Description: ***LINK_TO_6506A_Te5/1***

  !-<output omitted>-!

  5 minute output rate 5470000 bits/sec, 1025 packets/sec
     9311415126 packets input, 7136165402633 bytes, 0 no buffer
     Received 96568047 broadcasts (90433453 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 90433453 multicast, 0 pause input
     0 input packets with dribble condition detected
     9561095975 packets output, 9508345166932 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out
3750_DC_1#

Here you can see that there are no runts, giants or input errors, which is good. If there is a problem with media type mismatches, you will definitely get some errors.

  1. Further issues that may cause faults include incorrect TX and RX connectors. Try installing the connectors the other way around. Avoid this whenever possible as two emitters emitting at each other can damage the light sources. Just keep in mind that it can happen.

  2. Also, curving or coiling the fibre optic cable too tightly may cause excess refraction and cause errors or disconnection.

Most of the additional troubleshooting procedures for fibre optics beyond those explained by Rene have to do with the physical layer.

I hope this has been helpful!

Laz

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