Find out what MAC addresses are connected to each physical LAN port of router

I'm managing a router running Linux 3.10.87-rt80-Cavium-Octeon On the router's GUI, there's a "Switch ports" page where I can see all ethernet ports (4 LAN - 2 WAN) and their corresponding link negotiation info. However there's nowhere I can find which MAC address is connected to each physical port on the router. I've googled and searched here to see if there was a linux command that could achieve this but could not find anything. Any help is much appreciated.