Fibre Network Issues

Posted on 2013-10-01
Medium Priority
Last Modified: 2013-10-01
Hi all,

Before I start I need to say that I am not a networking person, and my knowledge is VERY limited. We have an issue setting up or fibre network between our two buildings in that, basically, it doesn't work!

We have had an 8 core fibre cable run between out two buildings and have had a telecoms contractor polish and present the cables into a fibre panel that allow 4 connections at each end. All of the cores have been tested and all have been passed by the engineers, so we know that the fibre works end to end.

Now, also at each end we have a Netgear FS728TPv2 switch that has 2x GigaBit SFP Ports, one of which we have inserted Cisco Tested mini-GBIC Transceiver plug-in module at each end.

We have then connected from this module to the fibre panel with LC/LC Duplex P/Cable. Same both ends.

Issue is that the two units are not communicating/connecting when connected via the fibre. If we connect via standard ethernet using one of the copper Gigabit ports all is well.

It is almost as if the units are not recognising that the fibre is connected, or that fibre is somehow disabled. I have read through all of the documentation on the switch and I cannot find anything about having to "enable" fibre.

Also, via the web interface to the switches, the cable status on the fibre ports is reported as "unknown", another indicator that something is wrong somwhere.

It is probably something stupid I am missing somewhere, because clearly the units are working ... as long as fibre is not used.

Sorry to ramble .... probably lack of sleep !!

Hope someone has some ideas as to what might be the issue.

Question by:matthewgreen
  • 2
LVL 47

Expert Comment

ID: 39536237
Swap the SFP's between known good lines and the questionable ones to determine whether or not one of them is bad.   Note SFPs generally allow only 50-100 re-insertions.
LVL 13

Accepted Solution

Daniel Helgenberger earned 1400 total points
ID: 39536238
I just shoot out one of the most common things:
- Can you see the 'light' from the TX'ing laser from the remote switch?
- Did you check for swapped RX/TX connectors?
Make sure you have the TX connector from one switch (the one with the 'light') pluged into the RX port of the SFP.

This this is all correct you need to check your SFP's and the swiches. The switch ports are usaually dual personality, this means they can either operate via Fibre or copper. Make sure there are no copper cables connected to these ports. Also, check the switches if the link is active.
I do not suspect it, but it may also be a spanning tree issure; so enable STP on both swiches.
LVL 47

Expert Comment

ID: 39536294
P.S. Do NOT look directly into the light.  There are two frequencies, short wave and long wave. One of them can blind you.  Let the light shine on your hand and look at it that way to be safe.

Author Comment

ID: 39536408
The issue was caused by the two pre-made bought fibre patch cables both being configured as "cross-over" cables, meaning that as Helge000 suggested we had swapped TX/RX connectors. With the help of the input and a few lines drawn on a sheet of paper we worked out that we only needed a cross-over at one end. Once we altered the configuration of the fibre patch lead at the other end all the lights came on.


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