By IP address in a packet received when using BRAS in L3 mode.
By PPPoE — a packet for authorization received from the subscriber’s PPPoE port (PADI). It allows you to identify a subscriber by login, mac-address, qinq tags.
By DHCP packet — from the broadcast DHCP Discovery packet that is received from the subscriber port in L2 mode, which allows getting information to identify the subscriber by mac-address or qinq-tag.