What if I do without the switch though...
From what I can tell, the #MoCA network functions as a switch. And assuming the FTTdp thing just bridges it all through, I'm basically connected to ISP router through a switch. So broadcasts and whatever junk will be sent to the ISP, which will probably drop the #VLAN junk, but the vast majority of the packets won't be sent there. So it'll work...
