The issue we had on site was that the fibre connections to the newly installed 3750 switches with GLC-LH-SM would either flap or not come up at all. We were replacing 3500Xls that had WS-G5486. Both sides had mode conditioning cables running over multimode, that had been in production for 4 years. Are there any known 3750 bugs, etc?
The supplied sfps/gbics were single mode long haul fibre gbics and the fibre patch leads were mode conditioning. The site originally had single mode long haul gbics (in most cabinets) however the patch leads were multi mode in most cases (from memory there was one mode condition cable). This was unusual given that the fibre backbone is 62.5 micron multimode and that the longest fibre run was no longer than around 250m (this is from memory of reading the site fibre audit). It would appear that the single mode GBIC'S are much more tolerant than the single mode fibre sfp's as they were working without any known issues even though single mode long haul gbics across a mutli mode back bone in theory shouldn't especially given the short distances that most cabinets had.
The fix on the day was to use short haul multi mode sfps and gbics and multi mode fibre patch leads. We had some extra multi mode fibre gbics and sfp's and had to purchase fibre patch leads on the day to get the site cutover. We also had to lend customer multi mode sfps from our lab.
I believe the problem was that it was assumed, and rightly so, that because the site had single mode long haul gbics originally, that replacing them with a like for like setup would be ok. I would have made the same assumption, however I had suggested through another email stream that a fibre audit would be a good idea prior to ordering the sfp's/gbics but I understand that it would be a difficult thing to do as this may require an outage to do so.
In summary, as it is a confusing issue, the structured fibre backbone is multi mode 62.5 micron with the longest fibre run , according to a fibre site audit, being no longer than around 250m. This would mean that short haul gbics/sfps with multi mode patch leads would be required and not long haul single mode as was supplied.