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. • 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.
Fundamental Concepts of Point-to-Point Serial Communication • Concept of serial communication as the basis of WAN technologies
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.
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.
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
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.
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.
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.
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
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.
Configuring HDLC Encapsulation • Router (config-if)# encapsulation hdlc
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.
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.
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.
Conti… • Purpose and format of each of the fields in a PPP frame
Point-to-Point Concepts • Three phases of PPP session establishment
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
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
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.
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.
Configuring PPP with Authentication • Differentiate between PAP and CHAP
Configuring PPP with Authentication • Describe how to use PAP to authenticate a PPP connection
Configuring PPP with Authentication • Describe how to use CHAP to authenticate a PPP connection
Configuring PPP with Authentication • Explain how to configure a PPP connection with authentication
Configuring PPP with Authentication • Explain the output of the debug ppp authentication command
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
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