PPP Overview

The Point-to-Point Protocol (PPP) is a layer 2 WAN protocol. PPP is used to connect a local area network (LAN) to a wide area network (WAN) through a service provider. PPP can be used as the data link layer protocol and frame for the following types of point-to-point, serial, synchronous and asynchronous network connections: Dial-Up, ISDN, Leased Line (e.g. T1 & E1), Frame Relay, DSL (PPPoE), and ATM (PPPoA). Concerning physical media, PPP can be transported over telephone lines, serial cables, fiber optic lines, twisted pair copper, satellite, and wireless.

For serial connections, Cisco routers by default use Cisco’s version of the HDLC protocol (Cisco-HDLC) but if you are going to connect to another non-Cisco router it is recommended to use the PPP protocol and frame encapsulation. The speed of PPP is limited only by the DTE and DCE interface connections it uses. It is capable of very fast speeds.

The Structure of PPP

The PPP protocol is based on, and similar to, the HDLC protocol with some important differences, in that PPP uses LCP, NCPs and supports: authentication, encryption and compression.

The PPP protocol structure


IP, IPX, AppleTalk

NCP (Network Control Protocol)
Upwards service to layer 3 (Network Layer)
multiple layer 3 protocol support through
NCPs. PPP can exchange layer 3 addressing

LCP (Link Control Protocol)
handles link setup, termination, authentication, encryption, compression, and error detection

PPP at the
Data Link Layer
PPP at the
Physical Layer
Synchronous or Asynchronous Serial
High Speed Serial Interface
Telephone Lines
Leased Lines

LCP – Link Control Protocol handles link setup and termination as well as most of the configuration options including: authentication, compression error detection, and multilink (load balancing)

Authentication – PPP can be configured to use PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol). Both protocols authenticate serial connections with a username and password but CHAP is more secure because it uses a three way handshake during the initial LCP link negotiation, and then uses three way handshake challenges at random times afterwards, insuring that the link is still trusted.
Compression – the CCNA curriculum covers two PPP compression configuration options: stacker and predictor. PPP can achieve better throughput by compressing data traveling across the links
Error detection – PPP can be configured to test and report the quality of the serial link, it can also detect interfaces in a loopback state.
Multilink – PPP can be configured to load balance across multiple serial interfaces for greater bandwidth.


Video Tutorials

In this video I configure PPP with PAP

In this video I configure PPP with CHAP

Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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