Hi,
strange behaviour I faced with.
Installing Multimode SFP S-85DLC05D with manual speed set 100M and Full-Duplex > other side CISCO SG350 with Mikrotik SFP S-85DLC05D. Works ok.
Installing Singlemode SFP S-31DLC20D with manual speed set 100M and Full-Duplex > other side CISCO SG350 with Mikrotik SFP S-31DLC20D. Does not work
Seems that the Multimode module implementation on speed selection (10 / 100 / 1000) is done and tested correctly or the SFP chipset has some limitation?!?!
I have only Mikrotik SFPs, so other brand's recommendation will not help.
1G connection speed works ok, but that's not the scope at this special case.

Cisco port shows activity LED and link ok, from WEB GUI as well (I guess that monitors the SFP and backplane comm itself and not the fibre link health).
At the same time checking from Mikrotik side ACT LED is off, no link observed, can't ping. (obviously)
I have latest OS 7.5 in CCR-1036-8G-2S+, and the test is done to prepare the 100M for a legacy Switch, but failed even with Mikrotik own module.

Is there a limitation with the Single mode module using in CCR-1036? I read that generally 100M SFP modules does not operates in SFP+ slots; which not the case.
The supported 1,25G SFP should operate on lower (and selectable speed) is SFP+ slot in my readings.
Did anybody meet this issue before?
Any advise?
Prior thanks.

I wasn't aware the S-85DLC05D could do 100M at all, and I can't find anything about it either. Same for the S-31DLC20D. So count your blessings if it works at all, I daresay. Where did you read that either SFP could do 100M?

I wasn't aware the S-85DLC05D could do 100M at all, and I can't find anything about it either. Same for the S-31DLC20D. So count your blessings if it works at all, I daresay. Where did you read that either SFP could do 100M?

No, I was not sure and still not till I read on Mikrotik's SFP compatibilty list that original (by designed, like SFPF1302C) 100M SFP modules have restrictions in SFP+ slots. That's fine with me.
Then I decided to use Mikrotik CCR with own Mikrotik SFPs as a "100 M fibre - copper conveter" (I know, that sounds weird, but anyway this unit will be router as well).
So I have all the SFPs and started to test. Multimode worked well on 100 and 1G and based on this positive experience I decided to use Single-mode (as this is the awaited scenario for existing OS2 fibre).

If this is not supported at all, then why the WEB GUI has the option available? See the SFP config > switching "off" the AutoNegotiation > 10 and 100 is available. For S-85DLC05D then make sense - tested.
So, this case the WEB GUI is wrong or the Singlemode SFP (S31-DLC20D) is "only 1G". Then WEB GUI must be changed accordingly.
Or, if module works on 10/100/1000, then something else is wrong.

Any idea?

SFP(+) is an unofficial standard and only specifies the mechanical and electrical interface. There are different protocols possible between the SFP port and the inserted module. Depending on speed and copper vs optical: MII, GMII, SGMII, raw 4b5b, raw 8b10b, and many more.
Technically, a SFP(+) host has no way to figure out what speeds and protocol a module supports. So it makes sense to offer everything possible and let the user try what works.

In case of MTs own SFPs, Mikrotik could hardcode this know-how into ROS and restrict the available options to what is actually supported by the SFP module. But this is not the case, at least not so far.