Connecting Fiber to switch takes down fiber link

[prouty]

Greetings -

I have a 3750 switch that has a fiber connection via media converter.  Converter and patch cables are new.  Fiber tests within tolerance for light.  Switch is configured for a flat VLAN and no special ACLs or security considerations are in play.  Every time the converter is patched into the switch the fiber link drops out.  Could there actually be a fault in the switch that could cause this?

[chrihussey]

Trying to get a clearer picture. You're connecting a copper port on the 3750 to a media converter for transmission across a fiber link? The converter says the fiber link is up and then when you complete the copper connection to the switch the fiber link drops?

1- What kind of 3750 port? (10/100 or 10/100/1000)

2- What is the brand of media converter?

3- What type of fiber? (MM (62.5or 50m) or SM)

4- What is the distance of the fiber run?

5- What is at the other end? Another fiber converter into another switch?

6- Have you verified TX/RX on the fiber for your connection?

[prouty]

in order asked:

1. 10/100/1000 set to auto negotiate

2.  I'm not sure I'm off site but both converters at each end have been replaced.

3. 62.5 MM

4. roughly 700' +/-

5. Yes convertor 'coppered' into a 3650 switch.

6. I have to assume the vendor working this for me at the site has tried that but cannot say yes for sure.  On that note though if they were reversed would the fiber-link lights on the converter light up at all?  The link looks good up to the point we plug into the suspect switch. 

UPDATE::  We put a 2960 in place as a quick workaround / proof of faulty device.  It is apparently something with the 3750 switch.  I am surprised / confused as I was under the impression that when using a media converter that it was the device's job to keep the fiber link alive and not the copper switch port.  I still don't understand how plugging into a switch-port caused the fiber-link to drop?!?!?  If I get an answer or root cause I will share here.

At any rate crisis is calm but reduced productivity status.  I am traveling to site Friday and will have to open a TAC case.

[chrihussey]

Glad to hear things are working.

It's rare, but if there is enough reflection on a fiber it could show a link as up even if there is a TX/RX reversal as it is only an indication of received light and not necessarily a good link.

[mamdohmardini]

my recommendation is that you have to replace the media converter with SFP, as the MC are not the optimal solution and they get faulty. 

BR

MTM