For your home router to establish a WAN connection to your ISP, a few things specific to PPP must happen first.
You may also have to do some research on LCP and NCP.
Check this link below, it may have the answer you are looking for (see the excerpt on PPPoE below):
PPPoE has two phases, the discovery phase and the session phase.•
Discovery: The client identifies the available servers.
To complete the phase the client and server must communicate a session-id. During the discovery phase all packets are delivered to the PPPoE control plane (CPM or MDA). The IOM identifies these packets by their ethertype (0x8863).
→ PPPoE Active Discovery Initiation (PADI). This broadcast packet is used by the client to search for an active server (Access Concentrator) providing access to a service.
→ PPPoE Active Discovery Offer (PADO): If the access server can provide the service it should respond with a unicast PADO to signal the client it may request connectivity. Multiple servers may respond and the client may choose a server to connect to.
→ PPPoE Active Discovery Request (PADR): After the client receives a PADO it will use this unicast packet to connect to a server and request service.
→ PPPoE Active Discovery Session-confirmation (PADS) A server may respond to the client with this unicast packet to establish the session and provide the session-id. Once the PADS was provided the PPP phase begins.
• Session: Once the session ID is established connectivity is available for the duration of the session, using ethertype 0x8864. Either client or server can terminate a session.
During the life of the session the packets may be uniquely identified by the client’s MAC address and session-id. The session can terminate either by PADT sent by the client or server or by an LCP Terminate-Request packet.