An SFP module has no link

You have pushed an SFP or SFP+ module into the cage, on both ends, and there is no link. /interface print shows the port, but without the R flag. You get the same picture with a DAC cable that one side accepts and the other does not.

This chapter is about a link that is not there at all. If the link does come up but at a lower speed than you expected, go to A wired port does not reach its speed. If the link keeps dropping and returning, look at temperature and optical power below first, and at the fibre itself after that.

The quick checks, in order

1. Does the cage see the module? /interface ethernet monitor sfp-sfpplus1 once. A good answer starts with sfp-module-present: yes, followed by sfp-vendor-name, sfp-vendor-part-number and sfp-wavelength. If it says no, there is nothing electrically: reseat it, try another module, and check that it is the right form factor.
2. What does the status say? In that same output: good is status: link-ok with a rate that makes sense. no-link means the module is present but the far end is not heard.
3. Is light arriving? sfp-rx-power in the same output. A healthy single-mode link sits roughly between -3 and -15 dBm. Around -40 dBm, or no reading at all, means nothing is coming in. If sfp-tx-power is normal and sfp-rx-power is dead, your side transmits but does not receive, which points at the fibre or the far end.
4. How hot is the module? sfp-temperature. Above seventy degrees modules start behaving strangely, especially in a closed cabinet.
5. Is the port enabled? /interface print where name~"sfp". A disabled cage never gives a link.
6. What does the log say? /log print where topics~"interface". Modules going in and out, and complaints about unsupported types, land there.

The usual causes, most common first

TX and RX are swapped

With duplex fibre in two separate strands this is the mistake everybody makes. Transmit on one side has to reach receive on the other. Swap the two strands at one end and try again. On an LC duplex connector you can usually pull the two halves out of the clip and turn them around.

The two modules do not match

Wavelength and fibre type have to line up: 1310 nm against 1310 nm, single-mode in single-mode fibre, multi-mode in multi-mode. BiDi modules deliberately use two different wavelengths and have to be ordered as a pair; two identical BiDi modules will never talk to each other.

Speed and auto-negotiation

A 1G module in an SFP+ cage does not always fall back by itself, and optical 1G links on RouterOS often want auto-negotiation=no on both ends. That is a line you set by hand: /interface ethernet set sfp-sfpplus1 auto-negotiation=no. The configurator does not write it.

Too much attenuation, or too little

A dirty connector, a sharp bend or a long run gives you too little light. The opposite exists too: two long-range modules with a short patch between them overload the receiver, and then you need an attenuator.

The far end refuses the module

Some vendors only accept their own coded modules. MikroTik does not play that game, but the switch on the other side might. The same goes for DAC cables between brands.

Copper where glass belongs, or the other way round

A cage is not an RJ45 socket. To get copper onto an SFP port you need a copper SFP module or a media converter, and a copper SFP usually does want auto-negotiation on.

What the configurator does about it

SFP and SFP+ cages appear as ordinary interfaces, under the names RouterOS uses: sfp1, sfp-sfpplus1 and so on. You can use them anywhere a copper port works: in the bridge, as a WAN port, as a trunk, as a bond member.

What the tool does not do matters more: the script writes no SFP settings at all. No auto-negotiation, no advertise, no sfp-rate-select, no shutdown temperature. The module is never touched. So the generated script is never the reason a module has no link, and there is no box in the tool that will fix one either.

The catalogue does know the speed of each cage, 1G for SFP and 10G for SFP+. That lets the network board say two useful things as soon as you draw cables: The speeds differ (10G and 1G): the cable runs at 1G, and for a cable between a copper port and an SFP port: Copper to SFP: this needs a copper SFP module or a media converter. That second one is exactly the kind of mistake you otherwise discover on site.

Not checked: which module you fit, what wavelength it is, how long your fibre is, whether the far end accepts it, and what arrives optically. The tool runs in a browser and sees no hardware. For that last part /interface ethernet monitor is your only source.

When it is not your router

The switch at the other end, the fibre in the ground, the splice in the cupboard and the media converter your ISP left. A second, short patch between two modules on your desk separates that in five minutes: if it links there, the fault is in the installation.

Read on: Bridge and ports, Checks about cabling and A wired port does not reach its speed.