@high-amp I may have assumed too much with my above scenario for your setup. While server is technically feeding the FMC, your ethernet connection to the FMC would come out of router, assuming you don't have switch.
A couple reasons why some use switches in addition to routers. One is they may have an excess of devices on network such that router doesn't have enough ports. Second is router is noisier environment than switch, therefore, switch is acting as filter, some may have better clocks and/or fiber out (Cisco switch mentioned above is one, UptoneEtherRegen another), and then adding the lps is adding to filtering effect. Also gets rid of complexity, getting rid of FMC and another ethernet cable.
Yes, the SFP modules need to be compatible, the OpticalRendu uses LC type connector, it does come with the unit. And yes, slowing down to 100Mbps and single mode are reportedly better, although the single mode not universally reviewed as superior. The purpose built FMC is the Sonore OpticalModule, presume this will result in further reductions in noise and provide better clocking vs. generic FMC's.
The reason I believe some find switches effective in their setup vs mine where I didn't is due to second ethernet port on my server. In my case router or noise is ONLY feeding my server, second port feeding first FMC completely filters network noise, the rerouting or detour required by adding switch only serves to complicate my network path, more direct path superior for me.I assumed cleaning network feeding prior to server would provide superior sound quality, in my case the added complexity trumped the lower noise floor resulting in more info but cookie cutter images, totally unnatural, and this with audiophile switch with built in lps. For those without second ethernet port on server, one is forced to use usb from server (usually noisy as it comes directly off motherboard) direct to dac or back to router or switch in order to add streamer, the router dirty, switch cleaner. For most comparing a second ethernet port out of server vs router or switch is impossible as their server's don't have the second ethernet port. I understand the point I'm making here is therefore moot to most, I keep reiterating the second ethernet port in threads because this was one of the most effective network optimizations I've experienced over the years I've been experimenting with network optimization. I would suggest for anyone looking to get a new server, and planning on running a separate streamer now or in the future they get one with second ethernet port! Another point here is that what's downstream of server is far more important than what's upstream, true in my experience and I've been told this from a number of personal conversations with the people at Uptone and Network Acoustics.