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: enter image description here

  • 2
    3rd party optics are MUCH less expensive, look around! – SpacemanSpiff Oct 9 '12 at 19:08

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.

  • Without knowing the cable type from Core to Switch, but knowing I bought a 62.5/125 patch cable to the switch -- Would that still work if I got an SX-SFP for the new switch? I know the old switch had a 50/125 patch cable. – armani Oct 9 '12 at 19:43
  • @armani I would advice you to stick with ONE cable type. But your core problem is that the SFPs from your core switch and your new switch don't match! Either connect SX to SX, or LX to LX. Anything else wont't work. So yes, I figure that getting an SX for your new switch would work. Check the SFPs. You can remove them from the switch - there should be a label on it, stating the wavelength. Compare, act accordingly. – Alexander Janssen Oct 9 '12 at 19:54
  • Tell us how it worked out, please. – Alexander Janssen Oct 9 '12 at 20:58
  • Will do. I have two non-Cisco SX SFPs I will try, but if they fail of course it may just be that Cisco hates non-Cisco SFPs. In which case it may take longer for me to report if a Cisco one worked because of the time to purchase/ship. – armani Oct 10 '12 at 13:24
  • As luck would have it, I found a Cisco-branded 1000BaseSX SFP module in our inventory! ^_^ And it did resolve the issue -- Now I have the users in this building connecting at Gigabit speeds. Thanks so much. – armani Oct 10 '12 at 19:27


Single mode and multimode fibres are not compatible. You'll have to change the optics so that you've got a matched pair.

You can, as SpacemanSpiff said, get 3rd party optics, but this isn't the place to make product recommendations, so that one's up to you.

OR, you can get a singlemode/multimode converter box.. But they're not cheap.

  • The core problem is not the different cabling, but that SX and LX won't work together. Different cabling might be another factor, but not the relevant one in this case. – Alexander Janssen Oct 9 '12 at 20:09
  • Either way. Unmatched technologies = No Chance In Hell. – Tom O'Connor Oct 9 '12 at 20:10

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