In some cases, billings systems require additional authentication data, like from which interface on the router client wants to connect.
PPPoE daemon can provide this information in attribute Called-Station-Id
called-sid=ifname|mac|ifname:mac By default is called-sid=mac Specifies how to represent Called-Station-ID. ifname - Called-Station-ID will contain name of interface accepted request. mac - Called-Station-ID will contain mac address of interface accepted request. ifname:mac - Called-Station-Id will contain both name and mac of interface.