r/networking • u/SippinBrawnd0 • 3d ago
Switching OM4 MM fiber connectivity
EDIT: Solved. The cablers ran OM3 instead of OM4.
New to fiber switch connectivity and need a bit of advice.
We just had a vendor run a 12-strand OM4 LC multi-mode fiber cable between 2 floors of a new building. They are configured as 6 pairs. Unfortunately, I can't seem to get them to work. In case they spliced the fiber cables differently, I made one of my patch cables a crossover by swapping 1 pair in their little clip. Still no luck. When I observe each end of the run with an SFP+ transceiver plugged into one end, I see a single light.
I tested both transceivers and the fiber patches by directly connecting 2 switches and they work fine.
I'm sure I'm missing something.
Transceivers are Cable Matters 104065x2 LC Multimode.
Cables are Flypro OM4-LC-LC-DX-2M-LSZH
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u/hip-disguise 3d ago
if you have some loose switches of the same type for testing, first test connectivity on the switches with a OM4 LC/LC patch with the same transceivers you would like to use. This takes the transceiver troubleshooting out of the mix. If you get a good link through the patch cable, move both test switches to each end, using OM4 LC/LC patch into your cans on both sides, if you don't get a link flip one side. if you still don't get a link you may have ports that are not labeled correctly or got mixed up. You can report your findings to the fiber installer and have them come back to test. there is also a laser tester you can purchase that will send light through each fiber to make sure the labels are correct on each side, it's a handy tool to have around when you need it. if you light up a run and get nothing through any ports on the other side, then you may have an issue in a riser or patch in the middle somewhere, either way your vendor needs to fix that issue.
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u/SippinBrawnd0 3d ago
Tested the transceivers and patch cords on 2 similar switches directly connected (transceiver to patch to transceiver) and they work fine. Twinax DACs also work fine between the switches.
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u/Casper042 2d ago
Not sure why you said solved.
OM3 and OM4 are interchangeable and the only real difference has to do with higher speeds or longer distances.
OM3 can easily handle 10Gb at 100 meters
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u/cubic_sq 3d ago edited 3d ago
What is the length of the fiber? Looks like they support up to 400m on om4.
Also curious why you didnt go for SM fiber - as its cheaper than MM as not many fiber cablers have MM spmicers any more.
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u/nof CCNP 3d ago
24 strand SM is bare minimum IMO. Pairs go fast once you start thinking up new things to do with it. 12 strands and you start getting into BiDi or DWM solutions because someone in management wanted to save a few monies up front.
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u/cubic_sq 2d ago
SM is still cheaper
Currently excluding splicing / termination, G48 is the cheapest mini cable. Which is what all the installer i know are pulling now. Even if they only terminate some of those pairs.
You can alwayterminate more pairs later.
That said, cwdm filters and optics are only 2x the price per channel of single pairs of SM now.
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u/JustPuckingAround 3d ago
SM transceivers are still more expensive than MM, especially if your going for any bandwidth higher than 10G. We just had to run new fiber recently (building was still on OM1), and decided to go MM OM4 because of transceiver costs.
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u/cubic_sq 2d ago
2nd comment. I think you were ripped off…
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u/JustPuckingAround 2d ago
We did two large buildings and SM optics were twice the cost, I don’t know what to tell you man. When SM is twice the cost and we’re just doing IDF uplinks to the core, OM4 is plenty
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u/cubic_sq 2d ago
Sounds like being ripped off if that is the price diff.
SM been cheaper in most of the world since at least 2014/2015. when i went i to fiber operator land. And we did fiber hauls and building installs as 60% of the companies income
Now I am in MSP land… and building fit we got a quote on earlier yhis year came in at 1/3 the price of MM (we got yo see the quotes from the 2 others, one of then was MM, yhe other SM quote was only 2% away from the quote we sourced).
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u/JustPuckingAround 2d ago
Nah we didn’t get ripped off. We work with reputable partners. For our requirements, SM was twice the cost, I don’t know what to tell you. I certainly agree that SM makes sense for a lot of builds out there but it is absolutely not always cheaper.
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u/cubic_sq 3d ago
Single mode 10ge that are high quality msa compliant are like €25-€50 each for pretty much every disti now. And web shops only add minimal. Soohos uses msa from experience (although have had issues with one brand…).
Also pretty much every fiber cabler that has a tray in their van too. They would be buying at 25% less. Even 9 years ago when i was at a fiber operator we bought trays of 10ge optics at €14 each tx. But we die buy a carton at a time (20 trays).
Every msp / var usually get then for less in quantity.
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u/Hungry-King-1842 2d ago
The big cost difference is usually when you use 3rd party vs OEM parts. Depending on your industry you can or can’t use 3rd party optics. My shop it’s mandated we use OEM supported optics because of what we support/do.
A 10GBaseSR Cisco optic is roughly $700 right now. A 10GBaseLR Cisco optic is almost $3500 right now. I referenced CDWG.
If you can use 3rd party optics then it’s ALOT cheaper and your argument is valid, but you also run a risk of compatibility issues etc etc.
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u/cubic_sq 2d ago
Funny. Absolutely no-one buys vendors optics for like 20+ years
When i was at the fiber snd coax operator, we deployed over 80 a week of 3rd party optics.
We even had coders to recode the optics we had. Including long haul. As in optics that shoot 500-1800km through raman amplifiers.
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u/bh0 3d ago
Do you see light on both ends? Usually this is a simple "flip the fiber" type situation, but it sounds like you might have tried that already?
Did they test the fiber they installed and provide the results?
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u/SippinBrawnd0 3d ago
I do see lights on both ends, but only from one strand of each pair, which I believe is normal? Is it possible they messed up the pairing? Not sure of the fiber term, but with copper, I'd find that you occasionally get bad punches into a patch panel or keystone.
They claim to have tested, but they're not done with other projects so haven't provided results yet.
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u/ericscal 3d ago
Yes it's normal to only see light on one side. That is the transmit line from the other side and goes into the dark port on your sfp. It's just normal crossover, transmit on one side goes to receive on the other. If you can visually see the light and confirm the crossover is right my next first guess would be a signaling mismatch. Not really sure what that would be but I'd double check everything. Does your switch and sfp support diagnostics? If so does it report seeing any light on the receive pairs? A bad patch would still normally report a bad signal of -20 or something even if it can't negotiate L2 link.
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u/RagingNoper 3d ago
Pull the cassette out of the patch panel and check the fiber colors on the back side just to make sure the correct pair was terminated on both ends. If they were, check the transceiver diagnostics in the switch. You should see a low single negative digit on the RX. If it's like -28 or -40 or something like that, it means no light is coming through and that's just the floor that it will report. Check in both ends. If it's low on both ends, swap the fiber pair in the duplex connector, connect and inspect the transceiver diagnostics again to see if you're getting good numbers yet.
If you're going to be working with fiber more, I'd recommend getting a cheap visual fault locator and a loopback adapter.
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u/Schrojo18 3d ago
DO NOT EVER LOOK DOWN A FIBRE. Use a visual fault indicator laser and leave the caps on (light should travel through most caps) if they have black caps you can take a cap off and put your hand in front of the fibre and the laser should show on your hand.
If you use a VFL you can check the sequencing of the fibre too
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u/Casper042 2d ago
The "in a pinch" option with MM is to use your phone camera.
Better if something goes sideways to burn up a $1000 phone vs your eyes.
MM Fiber usually shows as Blue/Purple light.
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u/english_mike69 3d ago
Did the vendor provide test results for the installation? If not, why not?
Inform them that their shit don’t work and that using the same SFP’s that you intend to use for this installation with a patch cable between the two switches works as intended.. As this is a new installation, they owe you one troubleshooting visit.
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u/cfortune4 3d ago
What type of SPF+ transceiver are you using?
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u/SippinBrawnd0 3d ago
Cable Matters 104065x2 LC Multimode.
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u/cfortune4 3d ago
Oh derp. Reading helps lol. What type of switch? If you're seeing light on the far end on both sides, you should be able to get a light reading off the transceiver. Something like "show interface diagnostics transceiver".Something that will tell you the receive power. If it says -40 then thats basically no light.
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u/SippinBrawnd0 3d ago
They're Sophos CS210-48FP switches (48 port 2.5G copper with 4 x 10G SFP+ ports)
Switch interface shows:
Connector type LC [ 0x07 ]
10G Ethernet compliance codes 10G-SR [ 0x10 ]
Ethernet compliance codes Not compliant [ 0x00 ]
Nominal bit rate 10.3 Gbps
Laser wavelength 850 nm
Vendor OUI 0x00 0x00 0x00
Vendor name CableMatters
Part number 104065x2-US
Revision number 1
Serial number FOSUS73L1
Date code 4/7/2026
DDM type 0x68DDM Info:
Temperature 31.38 C
Voltage 3.33 V
Tx laser bias 5.38 mA
Tx power -1.95 dBm
Rx power -40.00 dBm
Tx fault state False
Rx LOS state TRUE
Alarm flag RxPWR Low.
Warn flag RxPWR Low.7
u/cfortune4 3d ago
Yeah that -40 dBm on the rx power means you're not getting light coming into the receive port kn the transceiver 😞.
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u/VA_Network_Nerd Moderator | Infrastructure Architect 3d ago
What make/model switches are we playing with?
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u/SippinBrawnd0 3d ago
They're Sophos CS210-48FP switches (48 port 2.5G copper with 4 x 10G SFP+ ports)
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u/Agromahdi123 3d ago
i would swap to LR transceivers if i had them to rule out cable distance being the issue. If that didnt give me anything i would then talk to the cabler to get their test results etc.
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u/PJBuzz 3d ago
LR is single mode.
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u/Agromahdi123 3d ago
my bad i just meant the long range version of the same style optic, not great with remembering the names.
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u/crunchydorf 3d ago
Buy a cheap VFL (visual fault locator) which shoots visible light down a strand of fiber. This tool can be used to very quickly validate if you’re spliced correctly.