1. POINT-TO-POINT PROTOCOL packetlife.net
PPP Components PPP Summary
Link Control Protocol (LCP) Standard RFC 1661
Provides for the establishment, configuration, and maintenance of a
PPP link. Protocol-independent options are negotiated by LCP.
Asynchronous serial, synchronous
Interfaces
serial, ISDN, HSSI
Network Control Protocol (NCP)
A separate NCP is used to negotiate the configuration of each PPP Features
network layer protocol (such as IP) carried by PPP.
Protocol Multiplexing · Multiple NCPs
PPP Header Optional Authentication · PAP/CHAP
8 16 24 32
Optional Compression · Stacker/predictor
Address Control Protocol
Loopback Detection · Provided by LCP
LCP Header Load Balancing · Multilink PPP
8 16 24 32
Connection Phase Flowchart
Code Identifier Length
Auth Required
Dead Establish
Authentication Protocols
No Auth
Plaintext Authentication Protocol (PAP)
Original, obsolete authentication protocol which relies on the Terminate Authenticate
exchange of a plaintext key to authenticate peers (RFC 1334). Failure
Admin Success
Challenge Handshake Authentication Protocol (CHAP) Shutdown
Authenticates peers using the MD5 checksum of a pre-shared secret Network
key (RFC 1994).
Extensible Authentication Protocol (EAP) PPP Connection Example
Provides MD5-based authentication similar to CHAP (RFC 3748).
Could be expanded to support other EAP mechanisms as well.
General PPP Configuration LCP Configuration Request
LCP Configuration Ack
! Configure a peer account if authentication will be used
username peer-hostname password password CHAP Challenge
! Configure a local IP address pool if needed CHAP Response
ip pool name first-IP last-IP
CHAP Success
interface Serial0/0 IP Control Configuration Request
! Enable PPP encapsulation
encapsulation ppp IP Control Configuration Ack
! Enable CHAP and/or PAP for authentication
CDP Control Configuration Request
ppp authentication { chap | pap } [ chap | pap ]
! Enable compression CDP Control Configuration Ack
compress { predictor | stac }
! Enable peer IP address assignment (server side)
PPP Compression Algorithms
peer default ip address { pool name | IP-address }
! Enable IP address negotiation (client side) Stacker
ip address negotiated Replaces repetitive data with symbols from a
dynamic dictionary (more processor-intensive)
Multilink PPP Configuration Predictor
Attempts to predict sequential data (more
! Create the multilink interface
interface Multilink1
memory-intensive)
ip address IP-address subnet-mask
ppp multilink group group
Troubleshooting
show ppp multilink
! Assign physical interfaces to the multilink group
interface Serial0/0 debug ppp authentication
encapsulation ppp
ppp multilink group group
debug ppp { negotiation | packet }
by Jeremy Stretch v1.2