ITE I Chapter 6 1 Objectives Point-to-Point Protocol (PPP) Accessing the WAN Chapter 2 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. 2 Point-to-Point Communication Parallel and Serial Transmission Parallel transmission Achieves well below theoretical expectations Crosstalk interference Clock Skew - slows parallel transmission Cost of cable and synchronization Used for short paths Serial transmission used for all long-haul communications Fewer wires, cheaper cabling Transmit at higher, more efficient frequencies. 3 4 Serial Communication Standards Time Division Multiplexing (TDM) There are three key serial communication standards affecting LAN-to-WAN connections: RS-232 64 kb/s, 9-pin or 25-pin connectors used. common on PCs to connect modems used for console port on Cisco devices V.35 used by most routers that connect to T1 carriers. HSSI up to 52 Mb/s a DTE/DCE interface developed for high-speed communication over WAN links such as T3 lines. 5 TDM is a physical layer concept Transmits two or more channels over the same link by allocating a different time slot for the transmission of each channel. Examples: T1/E1 and ISDN (bit interleaving) 6
Statistical Time Division Multiplexing (STDM) Demarcation Point (demarc) STDM uses a variable time slot length allowing channels to compete for any free slot space More efficient than TDM 7 The interface between customer-premises equipment (CPE) and network service provider equipment 8 Location of DTE and DCE DTE - The CPE, generally a router 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. 9 Back-to-back router connections requires both DTE and DCE V.35 cables Set clock rate on DCE router 10 WAN Encapsulation Protocols HDLC Encapsulation The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections e.g. PSTN, ISDN Cisco HDLC is proprietary. Can only use between Cisco devices Cisco HDLC is the default encapsulation used on Cisco router serial interfaces R(config-if)# encapsulation hdlc Configure a Layer 2 encapsulation type appropriate for the type of WAN 11 R# show interface serial R# show controllers 12
Standard and Cisco HDLC Frame Format Troubleshooting the serial interface Serial x is down, line protocol is down Serial x is up, line protocol is down Frame contains a Protocol field to indicate the network protocol encapsulated. 13 Serial x is up, line protocol is up (looped) Serial x is up, line protocol is down (disabled) Serial x is administratively down, line protocol is down 14 Point-to-Point Protocol (PPP) PPP frame format Non-proprietary must use when connecting to non-cisco device Contains three main components: HDLC protocol for encapsulating datagrams over p2p links. Link Control Protocol (LCP) to establish, configure, and test the data link connection. Network Control Protocols (NCPs) for establishing and configuring different network layer protocols. 15 Flag = 01111110 Address = 11111111 Control Protocol Identify the network protocol being carried Data the specified protocol datagram FCS Check for bit errors 16 PPP At the physical layer, you can configure PPP on a range of interfaces, including: - Asynchronous serial - Synchronous serial - HSSI - ISDN PPP Layered Architecture LCP sets up the PPP connection and its parameters NCPs handle higher layer protocol configurations LCP terminates the PPP connection. 17 18
The three phases of PPP session establishment Characteristics of NCP LCP establishes the link 19 20 PPP options Authentication PAP or CHAP Compression available in Cisco routers are Stacker (LZS) and Predictor. Error detection specify the link quality threshold percentage, 1 to 100 Multilink (a.k.a. MP, MPPP, MLP, or Multilink) combines two or more channels to increase the WAN bandwidth PPP Callback PPP with Authentication Authentication is optional, but if used must be configured on both ends of the link with same password Two authentication protocols are available: PAP (Password Authentication Protocol) very basic two-way process username/password sent as clear text CHAP (Challenge Handshake Authentication Protocol ) three-way handshake uses a one-way hash function, typically MD5 periodic challenges protection against playback attack 21 22 PPP PAP and CHAP Authentication PPP Authentication CHAP has eliminated most of the known security holes Only configure PAP if the remote router does not support CHAP Authentication protocol is negotiated by LCP You may enable PAP or CHAP or both ppp authentication {chap pap chap pap pap chap } [callin] If authentication fails, LCP does not OPEN With CHAP, if a periodic authentication challenge fails the link is terminated. 23 24
Configuring PPP with Authentication Configuring other PPP options PAP authentication hostname R1 username R3 password sameone int s0/0 ppp authentication pap ppp pap sent-username R1 password sameone CHAP authentication hostname R1 username R3 password sameone int s0/0 ppp authentication chap 25 R(config)# interface serial 0/0 R(config-if)# R(config-if)# compress [predictor stac] R(config-if)# ppp quality percentage R(config-if)# ppp multilink R(config-if)# ppp callback [accept request] 26 27 28 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 29 30
Summary Lab 2.5.1 PPP authentication PAP 2 way handshake CHAP 3 way handshake Use debug ppp authentication to confirm authentication configuration After PPP configuration, use show interfaces command to display: LCP state NCP state 31 32 hostname R2 username R1 password cisco username R3 password cisco interface Serial0/0 ip address 10.1.1.2 255.255.255.252 no shutdown ppp authentication pap ppp pap sent-username R2 password cisco interface Serial0/1 ip address 10.2.2.1 255.255.255.252 clock rate 64000 no shutdown ppp authentication chap exit router ospf 1 network 10.2.2.0 0.0.0.3 area 0 network 10.1.1.0 0.0.0.3 area 0 exit end 33