Multimode LC connected to Catalyst 2950. Other end of cable connected to Catalyst 2900 Series X 100baseFX connector. When the two switches connect via fiber there seems to be no communication. The link lights do not turn on. Is there anything I'm doing wrong? Could the ports be dirty? Thanks in advance.

  • Did you try swapping the fibers on one end? You need the fiber to crossover between the switches.
    – Ron Maupin
    Commented Jun 24, 2016 at 15:47
  • No because they only fit in one way. If I pull out the fiber connectors from the FX connector I can see one has a red light going through it. That cable connects to the GBIC Transmit end on the 2950. The cable set only fits in one way on the FX port. It may be an SC connector, I'm not sure.
    – Matt Brown
    Commented Jun 24, 2016 at 16:00
  • The fiber connector will pop out of the SC housing, and you can reverse them. The end-to-end requirement is that Fiber A on one end needs to connect to Fiber B on the other end. Tx must go to Rx.
    – Ron Maupin
    Commented Jun 24, 2016 at 16:08
  • 1
    If the 2950 uses LC connectors, then it's a 1000b-fx. If the 2900 is really 100b-fx, there is a speed mismatch.
    – Ron Trunk
    Commented Jun 24, 2016 at 16:11
  • Thanks Ron Maupin, I will try reversing the fiber connector.
    – Matt Brown
    Commented Jun 24, 2016 at 16:43

1 Answer 1


The 2950's optical ports are gigabit -- 1000b-SX based on "multimode". That will not, ever, connect to a 100b-FX peer. They are different speeds, and different signalling protocols.

If your 2950 has SFP ports, you might be able to find a compatible 100bFX module for it.

  • My old 2950 has 2 GBIC ports.
    – Matt Brown
    Commented Jun 24, 2016 at 19:26
  • Exact model # of the switches? (very old 2950C's had FX ports) I am unaware of anyone still making GBIC FX modules. (SFP's, sure, but not full sized GBIC)
    – Ricky
    Commented Jun 24, 2016 at 19:34

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