Is the loop back test effective placed on switch fiber port when trying to determine if CRC errros are due to the faulty port or this is pointless as there is no real traffic?

  • What are you looping back? A single fiber from TX to RX, or ???
    – cpt_fink
    May 10, 2015 at 6:11
  • Looping with multi mode fiber on 850nm optic May 10, 2015 at 10:28
  • I don't know of a way to put much(any?) traffic on that link, you could link two optics together and test both ways. My first suggestion would be to clean the fiber patch cable.
    – cpt_fink
    May 10, 2015 at 19:05
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 11, 2017 at 16:42

1 Answer 1


Assuming this is an Ethernet link a loopback test would be useless unless you could also send some frames over the link. Unlike POS, frames are only sent when there is traffic to send and you won't see CRC errors if you don't have any frames.

There are devices supporting BERT (bit error rate test) so you can put a loopback fibre and run a test to determine if the optical module and framer is working correctly but it's not common on classic Ethernet switches.

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