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graham robinson

Fibre link shows "Connected" at one end but "Not connected" at the other


I have a strange problem I have not come across before, where one end of a fibre connection shows as "connected" but the other end shows as "not connected".

I have a C2960-48PST-L connected to a C3750X-12S-E via OM4 fibre. Both switches use GLC-SX-MM SFP's which show as present when you do a "show interface status".

The SFP on the 2960 has flashing link lights and shows as connected [line protocol is up, and "show interface gi0/1" command shows packets being sent but none received], but the interface on the 3750 shows as not connected. If I swap the fibre cores round both switches show as not connected. I have other 2960's connected to the 3750 with no problems, but these are in a different part of the building using OM3 fibre.

I have tried replacing the 2960 switch, replacing the fibre patch leads, and have tried different switch ports and fibre cores, but the symptoms always remain the same.

The only thing I can think is OM4 fibre is not compatible with GLC-SX-MM SFP's [as OM4 is not listed here in table 1 ], however everything I read online says OM3 and OM4 are fully compatible, and I have found a post here saying you can use GLC-SX-MM with OM4].

So I'm confused, can anyone suggest what may be causing this? Our SFP's do not have a support contract so Cisco will not confirm if they are valid with OM4 or not.




I don't think it's an incompatibility. The receive is what will give you a link up or down, and in the original state you had one side showing a link, so enough light was getting through. 


Now, you said when you swap fiber cores both sides show down. I assume you mean that you flipped the xmt and rcv. If this is so, did you flip them on both sides? If you only flip one side then neither side will come up because you will have xmt going to xmt, and rcv going to rcv. 


This sounds to me like a bad strand. what I'd suggest is take an LED flashlight and shine it through each strand, and verify on the other end you can see the light. It won't say much about the integrity of the strand but it will show whether it is broken or not.




The fact that it is OM4 fiber should have nothing to do with it. In the simplest terms, OM4 is the same as OM3 with the real difference being it's a higher grade with less impurities. That's why it can support runs of a greater distance. So that's not the issue.

It looks like you have replaced everything except the SFPs themselves and one of them may be bad. 

Hope this helps




Greetings Graham, Fiber Optics problems can be challenging to say the least, especially if you don't have access to the right tools for the job, such as Fiber Optics Visual Fault Locator (VFL), Power Meter, and OTDR.

One big problem for jumpers, adapters, and transceiver can be contaminates including dust and oils on the ends of the connectors which can create a variety problem to your links.

I use AFL Telecommunications one click cleaner, they make one SC, ST & FC and one for MU/LC connector that I use every time remove, replace jumpers or adapters. 

I'm fortunate to all tools I mentioned above, plus I splice fiber optics too, a lot IOSs have the "show interface transceiver" that gives you a lot useful information (examples below), but use a little bit caution in its conclusions, I've had a lot Network Engineers in the past swear up and down that "You have bad fiber and you need to re-splice it!" and usually it was a dirty end on the connector, either on the patch panel, patch cord, or even the SFP/ GBIC.    

Please post what your findings were (I love a happy ending!)!

Switch#show interface transceiver ?
detail Show interface transceiver detail
module Limit display to interfaces on module
properties Show interface transceiver properties
supported-list Show supported transceivers
threshold-table Alarm and warning threshold table
| Output modifiers

SW#show interface gig1/1 transceiver properties // shows temperature, optical power both transmit and receive, port voltages. // Shows transceiver details. !I use this one a lot.

Kind regards, Chuck 

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