Can anyone give some tips on S-31DLC20D on RB-760iGS with RX Lose condition?
I just got 2 sets of RB-760iGS with S-31DLC20D and connected them with a short 1m LC/LC duplex patch cord.
The SFP is detected, but no-link is ever established. The “RX Lose” field is ticked and the RX power is showing -40dB.
Have tried changing SFP modules and patch cords to new ones, no luck.
My questions are:
Does the SFP have polarity? Should I cross connect the patch cords? Mine is the dual straight LC-LC combined together.
I have tried using single cords and crossed the connection, but still no connection.
According to detailed specifications, available in table in Brochure (download link is from product page), it should be able to work with short patch cable: Tx power is between -3dBm and -9dBm while Rx power is between -24dBm and 0dBm. Which means maximum Tx power (-3dBm) is less than maximum allowed Rx power (0dBm).
The long range modules feature higher Tx power (e.g. range between 0dBm and +6dBm) and higher receiver sensitivity with lower Rx power (e.g. range between -35dBm and -10dBm), for those minimum Tx power is still higher than maximum Rx power and need optical attenuator if connected with short patch cables.
Duplex optical cables use one fibre strand per direction. AFAIK Tx and Rx side of SFP is standard (but should not matter if you’re using same model of SFPs on both ends of fibre) and can’t be changed (i.e. no MDI/MDIX like on copper). Using patch cord “straight” already effectively crosses fibres: of you turn both patch cord connectors in the same direction (e.g. away fom you) while keeping connectors horisontaly), then there’s “outer” strand (left strand on the left side becomes right strand on the right side) and “inner” strand. At the same time both SFP modules will have Tx on the same side (not sure which it actually is, let’s say it’s the left side), hence “outer” strand will connect left side of left SFP (Tx) with right side of right SFP (Rx). And vice versa for the “inner” strand.
You can try fixing bit rate in interfaces, auto negotiation might work or it might not.
Either your fiber lead is faulty, or yes, you need to cross your fiber leads on the patch lead. TX goes to RX and RX goes to TX. They are (or should be) crossed.
RX loose and a -40dB is essentially telling you that it is not receiving any light. So either the receiving end is broken, or the transmitting end is blown. SFPs should (and do) automatically adjust their power levels to not cause damage over short links. If you have 1G SFPs in 10G cages, that may cause an issue. It could even be dust AFAIK. As you have tried (according to your post), different SFP modules and different patch leads, it doesn’t sound like your issue is a hardware one, but rather a software one.
As mentioned before, try forcing duplex / speed settings on the SFP. I’ve seen that help before, especially when 1G modules are installed into 10G cages. You only mention a RB-760 - what’s on the other side of the optic? If it is proprietary, it may also very well be that the fiber is not transmitting light at the wavelength that you are expecting…
You said you have two sets of RB760iGS and SFP modules.. Do you have spare SFP modules? Or are you just swapping the SFP modules between the two routers?
If you are just swapping the two, I suspect you have a faulty SFP module.
Yes, duplex fibre has polarity, one is Tx, one is Rx, sometimes they need to be swapped on one end.
Yes, a short patch cable can damage long-distance SFP modules, the 10 and 20km modules don’t have this issue. Using short patch cables between 80km SFPs is asking for trouble.
So… Do you have three or more SFP modules? Or just two? Please carefully check that they are the same model.
Are you using the correct type of fibre cables? Single-mode vs multi-mode..
My first thought is that something is ‘broken’, either bad SFP or wrong cable.
Thank you all for the responses.
To answer the questions from the responses: I have tried cross connecting the cables, used different SFP modules (different brand), but still no connection.
The weird part is no matter which SFP module that I use, I don’t see any light coming out of the SFP ports as I would expect from a fiber device.
The SFP modules are all brand new. The RB760iGS are also brand new.
Is the light “that weak” that it is not “visible” enough by eye?
No, human eyes can’t see the single-mode wavelength.
Some can see the 850 nm multi-mode light though.
Try a cell phone camera for single-mode, may work, may not. Would appear as a small red light.
Your screenshot is showing Tx and Rx… I will ask again.. Specifically, which patch cable are you using? Can you link to the product you purchased. What colour is the jacket of the cable? Should be bright yellow..
Attached is the Mikrotik SFP one. The previous picture was from a different brand SFP modules.
Did you get all of them at the same time, from the same supplier?
Do you have any from an alternate supplier?
Did the cables come with a testing report?
Answer: Supplier is reliable as I have used their products before. I also have other cables direct from different factory.
Also, you said in you opening post, S-31DLC20D, but your screenshot does not show this..
Did the cell phone camera show the light?
Answer: YES, thanks for the clue. It turns out that it emits light at the SFP port, but it does not show at the end of the patch cord.
Still can’t believe this is cable issue since I have tried at least 7 different cables from 3 different manufacturers.
Should also be noted, I checked amazon.in, the cheapest cable there is more than 10x the link you posted..
Answer: different country, different trade mechanism, different platform can’t really compare eye to eye.
If you have actual S-31DLC20D modules, replace your cables, don’t get cheap, get quality..
