LX sfp minimum range

we are moving a router between 2 data centers and we only have LX sfp's for connection, is there any issue using LX sfp's in a short range deployment ?

A Cisco 1000BASE-LX optic has the following spec:


-3dBm maximum transmit power, -3dBm maximum receive. That means you
can run it over any length. (We use LX for everything.)

Theoretically speaking Yes there should be an issue while using the LX SFP
for short range because it may damage the receiver part. But we've been
using it for quite a long time within datacenter for rack to rack switch
connectivity without harming the SFP or the performance.


Aftab A. Siddiqui

I can confirm using LX SFP's for under 100' runs with no problems. Except for the one site that ordered Multi-Mode fiber...


vendors that specify a minimum distance for lx typically spec 2 meters.

even EX shouldn't spike the receiver at that distance as long as the max
RX is about +1.

You can put a 3dB or 5dB optical pad on the link if the receiver can't
handle zero-distance optical power.

We're using SFP LX for a couple of years even in back to back configuration
for equipments within the same rack with a 1 meter patch cord without any

Max TX is -3dBm, Max RX sensivity is -3dBm so there is no problem.

1. I don't think I've ever had a LX SFP that TX at -3 dBm, they're usually
around -5 to -7 dBm.

and example in a live router :

pymaunier@re1.tcr1.rb.par> show interfaces diagnostics optics ge-7/3/* |
match "Laser output power "
    Laser output power : 0.3160 mW / -5.00 dBm
    Laser output power : 0.1800 mW / -7.45 dBm
    Laser output power : 0.2600 mW / -5.85 dBm
    Laser output power : 0.3210 mW / -4.93 dBm
    Laser output power : 0.3070 mW / -5.13 dBm
    Laser output power : 0.3200 mW / -4.95 dBm
    Laser output power : 0.3180 mW / -4.98 dBm
    Laser output power : 0.3140 mW / -5.03 dBm

2. You can assume a patch cord add between 0.2 to 0.5 dB of attenuation so
even with a SFP TX at -3dBm, you won't receive at the Max RX sensitivity.

Awesome, i got some single mode LC LC fiber off monoprice, sounds like i
should be all set for this. Thanks for everyones info

- Jon

I have found that -5dB or -10dB attenuators must be used on the send or receive strands between Cisco LX connected switches at relatively short distances of < 1 km over standard singlemode fiber.

Other Vendors' SFPs rated up to 25 km do not need attenuators at distances <1 km.

SX can actually be a little more versatile. LX works only over single mode fiber. SX is designed to work over either. As long as you have SX at both ends, you can connect them with either single or multimode fiber as long as the fiber type is consistent over the entire run.

I believe you've got that backwards. See ciscos's sfp pages. Lx will go
550m on mm, 10k on sm. (though it doesn't tend to do that well on mm in
my experience. )

I sure did! Thanks for pointing that out.


Oups typo, RX from -3 dBm to -19 dBm

It's the contrary.
SX only works on multimode fibre, not on singlemode.

LX can work on both.

It can happends that SX works on singlemode but it can fail anytime.

LX over multimode fibre is documented on Cisco SFP/GBICs datasheets.


Cisco 1000BASE-LX/LH SFPGLC-LH-SM1SFP-GE-L2Operates on standard single-mode
fiber-optic link spans of up to 10 km and up to 550 m on any multimode

As I recall, the problem may not only be the power
(which can cause receiver saturation), but issue that
fibre paths shorter than (around) 2-10m do not properly
condition the light(*), which can result in some issues
at the receiver.


(*) My memory says modal distribution issues.
    While 'single mode' fibre only supports one
    mode of transmission, it takes a short distance
    for the fibre to really be single mode. You can
    use a mode filter to address the problem, or just
    use fibres that are at least a few meters.

When optics started to become scarce at various times, I've done a
number of back-to-back connections using SM fiber and have had zero
issues. I wouldn't even worry about it. Remember, many carriers won't
even touch MM and they aren't chronically reporting issues or going to
lengths to work around them.


In some enterprise applications, SX is "good enough" for the distances at
hand, and SX optics are cheap...

Yes, you are correct, I had them backwards in my head when I typed that.

On Behalf Of Pierre-Yves Maunier

just because you can doesn't mean you should.

we have experience multiple cases where LX-MMF-LX works great for 3-5+ years...
then one day no longer gets link. swapping to a different fiber pair restores link.
can't remember SX-MMF-SX failing after years of service.

That's why I wrote 'but it can fail anytime' meaning, I strongly recommand
to NOT do it.

I once tried an LX-SMF-MMF-LX type setup using mode conditioning patch
leads between the SMF-MMF and MMF-LX portions of the span. I would be
hesitant to recommend it, simply touching the patch lead on the MMF-LX
portion would result in horrendous error counts. Suffice to say, we bit the
bullet and got some SMF blown through (tubes ftw).

I have also used LX-LX on short SMF runs of a couple of metres, for 1G and
10G, with no issues.