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 Unresolved xref.

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.