Understanding PPP

PPP provides a standard method for transporting multiprotocol datagrams over a point-to-point link. PPP uses the High-Speed Data Link Control (HDLC) protocol for its physical interface and provides a packet-oriented interface for the network-layer protocols.

Internet Protocol Control Protocol (IPCP) (which negotiates for transport of IP version 4 datagrams), IPv6CP (which negotiates for transport of IP version 6 datagrams), the OSI Network Layer Control Protocols (OSINLCPs), and Multiprotocol Label Switching (MPLS) run within PPP.

The router supports dynamic PPP interfaces. For details, see Configuring Upper-Layer Dynamic Interfaces.

Framing

The software restricts the use of the general HDLC protocol (RFC 1662) to unnumbered mode:

• HDLC address field is 0xFF (all stations)
• HDLC control field is 0x03 (to indicate unnumbered mode)

The router does not support the following framing features:

• Numbered mode (RFC 1663)
• Autodetection of encapsulation

Error Frames

The router relies on higher-layer protocols to recover from PPP data loss. All unrecognized protocol data units (PDUs) are discarded; however, statistics are maintained for packets dropped.

Related Documentation

• Understanding PPP Link Control Protocol
• Broadband Remote Access Support for PPP Overview
• Configuring PPP over a Serial Interface