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Chabot College. Point-to-Point Protocol. Point-to-Point Protocol. Point-to-Point Protocol. successor to SLIP router-to-router host-to-network synchronous or asynchronous developed in the 80’s. Layering PPP Elements. PPP Operation. PPP Physical Interfaces.

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Chabot College

Point-to-Point Protocol

Point-to-Point Protocol

Point-to-Point Protocol

  • successor to SLIP

  • router-to-router

  • host-to-network

  • synchronous or asynchronous

  • developed in the 80’s

Layering PPP Elements

PPP Operation

PPP Physical Interfaces

PPP runs on the following types of WAN physical interfaces:

  • Asynchronous serial

  • ISDN

  • Synchronous serial

PPP Datagram Transmission

Encapsulation - supports HDLC protocol to provide encapsulation.

Link Control Protocol (LCP) - used to establish, configure, and test the data link.

Network Control Protocols (NCPs) - used to establish and configure different network layer protocols.

PPP Frame Example

PPP Link Negotiation

PPP stages

  • LCP frames configure and test the data link

  • The link is established, and facilities are negotiated.

PPP stages

  • NCP frames are sent to choose and configure network layer protocols.

  • The chosen network layer protocols (IP, IPX, etc.) are configured, and packets from each network layer protocol are sent.

LCP Options

PPP LCP Configuration Options

PPP LCP Configuration Options

  • PPP options:

    • Authentication

    • Compression

    • Error Detection

    • Callback

    • Multilink

  • Only Authentication is covered on the CCNA exam.

PPP Compression

  • Compression should only be used on low speed WAN interfaces (typically 56 Kbps and less).

  • When compression is performed in software, it might substantially degrade system performance.

    • Disable compression if the router CPU load exceeds 40 percent.

Compression Algorithms

  • Predictor - Determines whether the data is already compressed. If so, the data is just sent-no time is wasted trying to compress already compressed data.

  • Stacker - A Lempel-Ziv (LZ)-based compression algorithm looks at the data, and sends each data type only once with information about where the type occurs within the data stream. The receiving side uses this information to reassemble the data stream.

Compression Algorithms

  • MPPC - This protocol (RFC 2118) allows Cisco routers to exchange compressed data with Microsoft clients. MPPC uses an LZ-based compression algorithm.

  • TCP header compression - This type of compression is used to compress the TCP headers. (Good for Telnet)

PPP Multilink

PPP Authentication

PPP Authentication Overview

Authentication Phase (optional)

Takes place before entering the network-layer protocol phase.

PPP supports two authentication protocols:

  • PAP (RFC 1334)

  • CHAP (RFC 1334 and 1994)


  • simple

  • two-way handshake

  • done only upon initial link establishment

  • passwords are sent across the link in cleartext

  • remote node is in control of the frequency and timing of the login attempts



  • used at the startup of a link, and periodically

  • three-way handshake

  • the local router sends a “challenge” message to the remote node

  • local router (or a third-party authentication server such as TACACS) is in control of the frequency and timing of the challenges


Configuring Authentication

Configuring Authentication

Verifying PPP

  • Login