MikroTik

I have a device with combo port (CRS106) and I have noticed strange behavior. Steps to reproduce:

• connect combo's copper port to other device using patch cable
• leave combo's SFP cage empty
• set combo-mode=auto -> link is up and running using copper port

Now plug S-RJ01 module to the SFP cage. The device will select SFP despite the S-RJ01 module has no link -> the whole combo1 is now link down. How's that possible?

I don't have similar device, so my words are only "out of curiosity". I've seen this behaviour documented somewhere. Reasoning was in the line of "it's selection done down on bare hardware. If SFP module is plugged in, the data lines are taken away from RJ port and get allocated to SFP cage".

That means in case of broken link, no automatic failover will occur, but manual intervention is needed. Either remove SFP module or reconfigure combo-mode to copper.

Indeed. Combo ports are there to better utilize switch chip present in device (e.g. if one doesn't need SFPs, he can use additional RJs). You can think of them as if they were solely SFP ports and you could plug in either optical SFP or RJ45 port SFP but not both at the same time. Only that with combo ports you have the RJ45 SFP implicitly plugged in at all times when optical SFP is missing.

In case of needed link redundancy, use 802.3ad link aggregation or similar. Many possibilities allow to use different kind of physical media for individual members of a bond (depends on implementation in the link partner). In some cases, when all links are operational, bond capacity is combined capacity of all members. When one link fails, bond remains operational with reduced capacity.