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Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP). Accessing the WAN – Chapter 2. Objectives. Describe the fundamental concepts of point-to-point serial communication including TDM, demarcation point, DTE-DCE functions, HDLC encapsulation, and serial interface troubleshooting.

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Point-to-Point Protocol (PPP)

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  1. Point-to-Point Protocol (PPP) Accessing the WAN– Chapter 2

  2. Objectives • Describe the fundamental concepts of point-to-point serial communication including TDM, demarcation point, DTE-DCE functions, HDLC encapsulation, and serial interface troubleshooting. • Describe PPP concepts including PPP layered architecture, PPP frame structure, PPP session establishment, multiprotocol encapsulation support, link control protocol (LCP), network control protocol (NCP), and Internet Protocol Control Protocol (IPCP). • Configure PPP on a serial interface including enabling PPP encapsulation, verifying the PPP connection and troubleshooting encapsulation problems. • Configure PPP authentication including explaining PAP and CHAP authentication protocols, configuring PPP authentication using PAP and CHAP, and troubleshooting PPP authentication problems.

  3. Fundamental Concepts of Point-to-Point Serial Communication • Concept of serial communication as the basis of WAN technologies

  4. Serial Communication Standards • All long-haul communications and most computer networks use serial connections, because • the cost of cable and synchronization difficulties make parallel connections impractical. • The most significant advantage is simpler wiring. • Also, serial cables can be longer than parallel cables, because there is much less interaction (crosstalk) among the conductors in the cable.

  5. Time Division Multiplexing • TDM is a Physical layer concept. It has no regard for the nature of the information that is being multiplexed onto the output channel. • Statistical Time Division Multiplexing • To overcome the inefficiency of TDM, STDM was introduced as in TDM the slot remains empty if no data to transmit at the sender side.

  6. Describe the Fundamental Concepts of Point-to-Point Serial Communication • How two or more data streams are transported across a single physical connection using TDM

  7. DTE-DCE • A serial connection has a DTE device at one end of the connection and a DCE device at the other end. The connection between the two DCE devices is the WAN service provider transmission network. In this case: • The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network. • The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link. This signal is received at the remote DCE, which decodes the signal back into a sequence of bits. The remote DCE then signals this sequence to the remote DTE.

  8. HDLC Encapsulation • WAN Encapsulation Protocols • To ensure that the correct protocol is used, you need to configure the appropriate Layer 2 encapsulation type. The choice of protocol depends on the WAN technology and the communicating equipment. • HDLC - The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. • PPP - Provides router-to-router and host-to-network connections • Serial Line Internet Protocol (SLIP) - A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.

  9. Conti… • Frame Relay - Industry standard, switched, Data Link layer protocol that handles multiple virtual circuits. Frame Relay is a next generation protocol after X.25. • ATM - The international standard for cell relay in which devices send multiple service types (such as voice, video, or data) in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays.

  10. HDLC Encapsulation • Flag - The flag field initiates and terminates error checking. The frame always starts and ends with an 8-bit flag field. The bit pattern is 01111110. • Address - The address field contains the HDLC address of the secondary station. This address can contain a specific address, a group address, or a broadcast address. A primary address is either a communication source or a destination, which eliminates the need to include the address of the primary. • Control - The control field uses three different formats, • Information (I) frame: I-frames carry upper layer information and some control information. • Supervisory (S) frame: S-frames provide control information. An S-frame can request and suspend transmission, report on status, and acknowledge receipt of I-frames. • Unnumbered (U) frame: U-frames support control purposes

  11. Protocol-(only used in Cisco HDLC) This field specifies the protocol type encapsulated within the frame • Data-The data field contains a path information unit (PIU) or exchange identification (XID) information. • Frame check sequence (FCS)-The FCS precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder.

  12. Configuring HDLC Encapsulation • Router (config-if)# encapsulation hdlc

  13. Trouble shooting the Serial Interface • show interfaces serial • show controllers • Cisco 7000 series routers use a cBus controller card for connecting serial links. With these routers, use the • show controllers cbus command.

  14. PPP • What is PPP? • Recall that HDLC is the default serial encapsulation method when you connect two Cisco routers. • With an added protocol type field, the Cisco version of HDLC is proprietary. • Thus, Cisco HDLC can only work with other Cisco devices. • However, when you need to connect to a non-Cisco router, you should use PPP encapsulation.

  15. Conti… • PPP encapsulates data frames for transmission over Layer 2 physical links. • PPP establishes a direct connection using serial cables, phone lines, trunk lines, cellular telephones, specialized radio links, or fiber-optic links. • There are many advantages to using PPP, including the fact that it is not proprietary. • The link quality management feature monitors the quality of the link. If too many errors are detected, PPP takes the link down. • PPP supports PAP and CHAP authentication.

  16. Conti… • Purpose and format of each of the fields in a PPP frame

  17. Point-to-Point Concepts • Three phases of PPP session establishment

  18. PPP Configuration Options • PPP can be configured to support various functions including: • Authentication using either PAP or CHAP • Compression using either Stacker or Predictor • Error Detection • Multilink which combines two or more channels to increase the WAN bandwidth

  19. PPP Configuration Commands • R3#configure terminal • R3(config)#interface serial 0/0/0 • R3(config-if)#encapsulation ppp • Compression • R3(config-if)#compress [predictor | stac] • Quality • R3(config-if)#ppp quality 80 • Multilink • Router(config-if)#ppp multilink –this command perform load balancing • Verification-- show interfaces serial • Debug-Packet, negotiation, authentication, compression, etc

  20. PPP Authentication Protocol • Password Authentication Protocol (PAP) • Initiating PAP PAP provides a simple method for a remote node to establish its identity using a two-way handshake. • Command is ppp authentication pap • username name password password • It must match the user name and password of the other router.

  21. Conti… • Challenge Handshake Authentication Protocol (CHAP) • Unlike PAP, which only authenticates once, CHAP conducts periodic challenges to make sure that the remote node still has a valid password value.

  22. Configuring PPP with Authentication • Differentiate between PAP and CHAP

  23. Configuring PPP with Authentication • Describe how to use PAP to authenticate a PPP connection

  24. Configuring PPP with Authentication • Describe how to use CHAP to authenticate a PPP connection

  25. Configuring PPP with Authentication • Explain how to configure a PPP connection with authentication

  26. Configuring PPP with Authentication • Explain the output of the debug ppp authentication command

  27. Summary • PPP is a widely used WAN protocol • PPP provides multi-protocol LAN to WAN connections • PPP session establishment – 4 phases Link establishment Link quality determination Network layer protocol configuration negotiation Link termination • WAN Encapsulation • HDLC default encapsulation • PPP

  28. Summary • PPP authentication • PAP • 2 way handshake • CHAP • 3 way handshake • Use debug ppp authentication to confirm authentication configuration • PPP configuration • Done on a serial interface • After PPP configuration, use show interfaces command to display: • LCP state • NCP state

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