Point-to-Point Protocol (PPP)

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1

Point-to-Point Protocol (PPP)

Accessing the WAN – Chapter 2

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 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.


Conti… 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. STDM is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. Using STDM allows bandwidth to be split over 1 line. Many college and corporate campuses use this type of TDM to logically distribute bandwidth.

If there is one 10MBit line coming into the building, STDM can be used to provide 178 terminals with a dedicated 56k connection (178 * 56k = 9.96Mb).

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


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 contains 3 main components :

HDLC for encapsulating L3 over point to point links.LCP to establish, configure & test L2 connection.Family of NCPs for configuration different protocols.

LCP LCP sits on top of the physical layer & establishes configures & tests the data link connection.LCP establishes the P2P link, negotiates & sets up control options on the WAN data link.LCP provides the following services:Handling varying limits on packet sizeDetecting common misconfiguration issuesTerminating the linkDetermining when link is functioning or notSetting up authentication compression & error detection


Conti… NCP

PPP permits multiple L3 protocols on the same linkFor every L3 protocol used PPP uses a separate NCPe.g. ICPCP, CDPCP, IPXCP, ATCPEach NCP manages the specific needs required by its respective networklayer protocols


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

Link Quality Monitoring The percentages are calculated for both incoming and outgoing

directions. The outgoing quality is calculated by comparing the totalnumber of packets and bytes sent to the total number of packets andbytes received by the destination node. The incoming quality iscalculated by comparing the total number of packets and bytes receivedto the total number of packets and bytes sent by the destination node.

If the link quality percentage is not maintained, the link is deemedto be of poor quality and is taken down. Link Quality Monitoring (LQM)implements a time lag so that the link does not bounce up and down.


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 phasesLink 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


Point-to-Point Protocol (PPP)

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