As has been pointed out by Jörg, the 'router' above can very well be an 元 switch that is used as a router. Of course, both NICs could be in different VLANs since those represent separate segments. The number of switches in between doesn't matter - each broadcast is propagated throughout the broadcast domain (=L2 segment), so each NIC messes up the source-address table for the other on every participating switch. For a router to see identical MAC addresses in different subnets is not normally a problem. Also, they would have to be in different IP subnets to enable normal routing. The NICs need to be in different L2 segments - at least one router in between. What is the least possible separation (in terms of number of switches, routers, different IP subnets etc.) needed that would still allow traffic between the NICs? Identical MAC addresses will disable reliable switching/bridging. You can't have that within the same link-layer segment. Suppose you have two NICs with the same MAC address, but not necessarily the same IP address.

Guest network mac address issues