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Chapter 3 -- PPP. PPP – A continuation of CCNA – Semester 4. DataLink (Layer 2) TCP/IP Protocols. SLIP - SLIP is a standard protocol for point-to-point serial connections, using TCP/IP. SLIP was a predecessor of PPP.

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Chapter 3 ppp

Chapter 3 -- PPP

PPP – A continuation of CCNA – Semester 4

Datalink layer 2 tcp ip protocols
DataLink (Layer 2) TCP/IP Protocols

  • SLIP - SLIP is a standard protocol for point-to-point serial connections, using TCP/IP.

  • SLIP was a predecessor of PPP.

  • PPP - PPP provides router-to-router and host-to-network connections over synchronousand asynchronous circuits, which can be either dialup or leased lines.

Ppp supports
PPP Supports

  • Protocols – TCP/IP, AppleTalk, IPX

  • Essential features such as dynamic address allocation, PAP authentication, CHAP authentication, and Multilink PPP


  • High-Level Data Link Control (HDLC) is the default encapsulation for ISDN and serial interfaces on a Cisco router. Although HDLC is a default encapsulation, Cisco's HDLC is not necessarily compatible with other vendors' HDLC implementations

  • PPP is a standard protocol – can be used with any vendor equipment

Ppp specifications
PPP Specifications

  • It is a standard – operates at OSI Layer 2

  • Encapsulates Layer 3 datagrams with a specialized frame.

  • PPP defines the Link Control Protocol (LCP)

  • Once the LCP establishes the Layer 2 connection, the Network Control Protocol (NCP) takes over– frame includes protocol field

    • Each Layer 3 protocol has its own NCP. For example, IP's NCP is IPCP; IPX's NCP is IPXCP, and Appletalk's NCP is ATALKCP


  • RTA(config)#interface async 2

  • RTA(config-if)#encapsulation ppp

Remote connections
Remote Connections

  • Exec

    • Can dial in and take control of command line

      • Telnet

  • Access resources – network access

    • Can access printers, servers, etc.

      • PPP encapulation via asynchronous dialup connection


  • RTA(config)#interface async 1

  • RTA(config-if)#encapsulation ppp

  • RTA(config-if)#async mode interactive

  • RTA(config)#line 1

  • RTA(config-line)#autoselect ppp during-login

    • Will eliminate possibility of Exec session

  • Router(config-if)#async mode dedicated

    • Will ensure PPP runs on specified line

Programming interface
Programming Interface

  • RTA(config)#interface async1

  • RTA(config-if)#ip address10.1.1.1 or

  • RTA(config-if)#ip unnumbered lo0

  • RTA(config-if)#peer default ip address

    • Assigns an address (usually host on same subnet) to the dial-in unit or

  • RTA(config-if)#peer default ip address pool NAME

    • Requires global command # ip local pool pool-name starting-address end-address.

Lcp configuration options
LCP Configuration Options

  • Authentication

    • CHAP or PAP

  • Callback

    • Billing consolidation

  • Compression

    • Reduces size of frame - Stacker, Predictor, and Microsoft Point to Point Compression (MPPC)

  • Multilink PPP

    • load balancing functionality over multiple WAN links

Authentication programming
Authentication Programming

  • Router(config)#username Romeo password Juliet

  • Router(config-if)#encapsulation ppp

  • Router(config-if)#ppp authentication pap

  • Router(config-if)#ppp pap sent-username GIN password RUMMY

    • Name and password are case sensitive

  • Dialer-map command tells each router what to do (suggest it be used) – see 3.2.2

Configuring chap
Configuring CHAP

  • Router(config-if)#encapsulation ppp

  • Router(config-if)#ppp authentication chap

  • Router(config-if)#ppp chap hostname GIN

  • Router(config-if)#ppp chap password RUMMY

    • Remember that you will also have to configure a local username/password database, or point the router to the TACACS+ or RADIUS server that has that information.

    • Router(config-if)#ppp authentication pap chap


  • provides a client/server relationshipbetween the endpoints of a point-to-point connection

  • can be used to control access and toll costs between hosts

  • the calling router (the callback client) pases authentication information to the remote router (the callback server)

  • for PPP callback server disconnects, and then places a return call

  • Both routers on a point-to-point link must be configured for PPP callback – server/client

Configuration for callback
Configuration for Callback

  • Server(config)#username Client password itsasecret

  • Server(config)#map-class dialer DIALBACK

  • Server(config-map-class)#dialer callback-server username

  • Server(config-map-class)#exit

  • Server(config)#interface async 1

  • Server(config-if)#ppp callback accept

  • Server(config-if)#dialer map ip name Client class DIALBACK modem-script hayes56k broadcast 5556002

Client configuration
Client Configuration

  • Client(config)#interface async 1

  • Client(config-if)#encapsulation ppp

  • Client(config-if)#ppp authentication chap

  • Client(config-if)#ppp callback request

  • Client(config-if)#dialer in-band

  • Client(config-if)#dialer-group 1

  • Client(config-if)#dialer map ip name Server modem-script hayes56k broadcast 5556001


  • CPU or Memory Intensive

    • Can impact router performance

  • Use only on slow lines

  • Use commands to show impact

    • Show process cpu

    • Show processes memory

Ppp multilink
PPP Multilink

  • Use MLP with applications in which bandwidth requirements are dynamic, such as remote LAN access applications for telecommuters or small office, home office (SOHO) environments

  • Creates bundles

  • RFC 1717 and 1990

Mmp features
MMP Features

  • Combine multiple physical links into one logical link (bundle)

  • Receive and reassemble upper-layer protocol data units (PDUs)

  • Receive PDUs of a negotiated size

Troubleshooting commands
Troubleshooting Commands

  • Show dialer

    • view the status of asynchronous dialup connections.

  • Debug PPP negotiation

    • troubleshooting the PPP LCP activities such as authentication, compression, and MLP

  • Debug PPP Authentication

    • output is limited to CHAP and PAP authentication

    • events.