I'm trying to replace an old Cisco Catalyst 2950 with a new Cisco 3750X. Unfortunately, these SFP's are so expensive, I'm trying to work with what I've got. Please note the diagram to see what connections I have. The core switch is SX, as is the old switch it was running to. When I hook a cable [had to be a different end connector to fit the new SFP, so it's really a different cable but attached to the same patch port] to the new switch, nothing happens and it reports the line is down [Physical Layer error]. This port reports that it is a 1000BaseLX, which from what I understand is Single-Mode fiber, instead of Multi-Mode like the rest of this network.
Knowing almost nothing about fiber optics, what would you propose is the easiest way to remedy this situation, getting connectivity to the new switch? I've heard that over short distances, a Single-Mode connection will work on a Multi-Mode cable -- Is this true?
Thanks in advance for your thoughts.
Edit: Of course I forgot the diagram: 
Answer
SX and LX is not about single- and multimode cables. It's different wavelengths of the Laser being used. LX has a much longer wavelength than SX.
SX wavelength is usually 850 nm, where as LX is usually around 1300 nm.
There is still a difference between multi- and singlemode cabling, but since you seem to be having a legacy-cabling, I bet you're using multimode anyway - which is compatible with SX and LX.
You may use regular 50/125 cables for both wavelengths, but both ends have to match the laser's colour.
So the problem is not necessarily your cabling (unless it's old 62.5/125 cabling, where cabling-length restrictions apply), but that the colour of your SFPs simply don't match. However, if your cabling is 62.5/125, you need to use patchcables of the same dimensions at both ends. So do not mix both multimode cable types. It may work, but I wouldn't count on it.
Either get your Core-Switch a new SFP (LX SFPs are really cheap nowadays) or replace your new LX-SFP by a regular SX-SFP.
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