strange behavior with fiber optic cable - port doesn’t come up

[softcake]

Hi Community,

At our client switch (C2960X) the uplink-port is shown as "up". But on the aggregation switch (3750X) the port is "down".
cdp neighbor shows no switch. I have already exchanged the SFP's, but without success. I have also changed the ports already. Nothing.
But if I connect the line to another switch of type 2960X, both ports go online. If I use another 3750X, the port on the 3750X stays down, too.
I have tested with different SFP's (LH-SMD & MM-SMD). The cabling consists of a multimode fiber.

2960X:

GigabitEthernet1/0/51 is up, line protocol is up (connected)

3750X:

GigabitEthernet2/0/11 is down, line protocol is down (notconnect)

Does anybody have an idea what this could be due to?

[Leo Laohoo]

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@softcake wrote:

At our client switch (C2960X) the uplink-port is shown as "up". But on the aggregation switch (3750X) the port is "down".

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The fibre run is faulty:  One side has both pairs working and the other side only has half the pair working.  

[softcake]

I'm not sure with that. I checked both pairs with a laser and both sent light to the other end.

And why, when one line of the pair is faulty, does the ports come up, when I connect them to two switches of the same model?

[Georg Pauwen]

Hello,

can you try and switch the transmit and receive ends (on either side) ? Not sure if that is a possibility with the cable and SFPs you have...

[softcake]

already changed both ends. But same behaviour.

the line had also been working perfectly for months. Only after a manual shutdown of the port on the 3750X last week did it not come up again.