Single mode VS Multimode in a server room? I do fiber internally for the company I work with. The only fiber I dont touch is Comcast in our buildings. Anyway, we recently switched to single mode and when talking to the LeCom guy (they do Comcast terminations for the most part) they were telling me they normally used multimode for runs that are short because single mode will burn out the optics over time if it doesn't have an LC Attenuator (we use lc to lc everything). Should we be using Attenuator? Or just use multimode from the switch to switch connections that are short?

The data sheet "Cisco SFP Modules for Gigabit Ethernet Applications Data Sheet" says "the minimum cable distance for all SFPs listed (multimode and single-mode fiber) is 6.5 feet (2 m)."

However even though the transmit maximum power is equal to the receive maximum power, for very short distances (5m) I get receive overload warnings which may be due to reflections (see my question "1000BASE-LX/LH Receive power level greater than transmit power level" )

The datasheet text quoted in the other answer is for the "1000BASE-ZX SFP for Long-Reach Single-Mode Fibers" which have a higher transmit than receive power

The 1000BASE-LX/LH can use SM or MM, however mixed SM and MM cable in the fixed plant causes me a lot of headaches and I would recommended SM with attenuators if needed.