None I can think of. The PPPoE server can be bound to one interface - LAN. PPPoE packets can be filtered in firewall if needed. You can read about Bridge here: http://www.mikrotik.com/testdocs/ros/2.9/interface/bridge.php I think you should filter PPPoE packets so that they do not go upstream and also use additional filtering to protect the upstream link from junk/unwanted traffic.