PPPoE

PPPoE is defined in RFC 2516 and brings the dial-up era's PPP protocol to Ethernet-based access networks. A connection is set up in two stages. During discovery the client broadcasts PADI and the access concentrator (BNG/BRAS) answers with PADO. The client requests with PADR and the server assigns a Session ID with PADS. PADT tears the session down. Discovery and session frames use EtherTypes 0x8863 and 0x8864. PPP then negotiates LCP, authenticates with PAP/CHAP and assigns addresses via IPCP/IPv6CP.

The BNG forwards the credentials to a RADIUS server, and values such as speed profile, static IP and session time are applied from the RADIUS reply. Because subscribers are identified by username regardless of the physical line, PPPoE is widely used on xDSL and fiber networks in Türkiye as well as by wireless ISPs (WISPs). On fiber services the PPPoE client usually runs on the ONT or on the router behind it.

The PPPoE header adds 8 bytes per packet, so on a standard 1500-byte Ethernet MTU the PPPoE MTU is 1492 bytes. If MTU and TCP MSS (typically 1452) are not set correctly, hard-to-diagnose symptoms appear, such as certain websites failing to load. Networks that support RFC 4638 can keep a full 1500-byte MTU using baby jumbo frames. The alternative, IPoE (DHCP-based access), has less overhead but needs another way to identify subscribers, such as DHCP Option 82 or per-subscriber VLANs.