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

Point-to-Point Protocol (PPP)

Accessing the WAN – Chapter 2

Glenn Avendano

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Describe the Fundamental Concepts of Point-to-Point Serial Communication If parallel is faster than serial, is parallel more suitable

for connecting to a WAN? In reality, it is often the case that serial links can be clocked considerably faster than parallel links, and they achieve a higher data rate, because of two factors that affect parallel communications: clock skew and crosstalk interference.

The more wires one uses and the farther the connection reaches, compounds the problem and adds delay. The need for clocking slows parallel transmission well below theoretical expectations. Most serial links do not need clocking.

Since serial cables have fewer wires, there is less crosstalk, and network devices transmit serial communications at higher, more efficient frequencies.

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Serial Communication Standards RS-232 - Most serial ports on personal computers conform

to the RS-232C or newer RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used. Many network devices use RJ-45 connectors that also conform to the RS-232 standard.

V.35 - Typically used for modem-to-multiplexer communication, this ITU standard for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. In the U.S., V.35 is the interface standard used by most routers and DSUs that connect to T1 carriers.

HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mbps. Engineers use HSSI to connect routers on LANs with WANs over high-speed lines such as T3 lines.

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Explain how two or more data streams are transported

across a single physical connection using TDM

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Describe the Fundamental Concepts of Point-to-Point Serial Communication

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Define the location of the demarcation point relative to

customer and service provider networks

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Explain the terms DTE and DCE with relative to the

location of devices in a network

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Explain the terms DTE and DCE with relative to the

location of devices in a network

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Describe how high-level data link control (HDLC) uses

one of three frame types to encapsulate data

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Describe the Fundamental Concepts of Point-to-Point Serial Communication

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Cisco HDLC is the default encapsulation method used

by Cisco devices on synchronous serial lines.

One may use Cisco HDLC as a point-to-point protocol on leased lines between two Cisco devices. If you are connecting to a non-Cisco device, use synchronous PPP.

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Describe the Fundamental Concepts of Point-to-Point Serial Communication Click the status button on page 2.1.7.1 to identify the

following five possible problem states in the interface status line: <from sh int serial command>

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Activity

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Activity

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Activity

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Describe Point-to-Point Concepts HDLC is the default encapsulation method between

Cisco routers. Use PPP to connect to a non-Cisco router.

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.

PPP components: HDLC protocol for encapsulating datagrams over point-to-point links; Extensible Link Control Protocol (LCP) to establish, configure, and test the data link connection; family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols.

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Describe Point-to-Point Concepts PPP Architecture: Physical Layer

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Describe Point-to-Point Concepts The LCP provides automatic configuration of the interfaces at each end, including:

Handling varying limits on packet size

Detecting common misconfiguration errors

Terminating the link

Determining when a link is functioning properly or when it is failing

–PPP also uses the LCP to agree automatically on encapsulation formats (authentication, compression, error detection) as soon as the link is established.

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Describe Point-to-Point Concepts PPP permits multiple network layer protocols to operate

on the same communications link. For every network layer protocol used, PPP uses a separate NCP.

NCPs include functional fields containing standardized codes to indicate the network layer protocol that PPP encapsulates.

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Describe Point-to-Point Concepts Describe the purpose and format of each of the fields in

a PPP frame

Roll over the packet fields on page 2.2.3.1 to view descriptions.

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Describe Point-to-Point Concepts Define the three phases of PPP session establishment

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Describe Point-to-Point Concepts PPP can be configured to support various functions

including: Authentication using either PAP or CHAP, Compression using either Stacker or Predictor, Multilink which combines two or more channels to increase the WAN bandwidth.

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Activity 2 (page 2.2.6.2)

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Activity 4 (page 2.2.6.2)

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Configure PPP on a Serial Interface

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Configure PPP on a Serial Interface Explain the output of the show interfaces serial

command

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Configure PPP on a Serial Interface The no form of this command disables debugging

output.

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Configure PPP on a Serial Interface

The no form of this command disables debugging output.

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Configure PPP on a Serial Interface The figure shows the output of the debug ppp

negotiation command in a normal negotiation, where both sides agree on network control program (NCP) parameters. In this case, protocol type IP is proposed and acknowledged.

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Configuring PPP with Authentication PPP Authentication – RFC 1334

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Configuring PPP with Authentication PAP is not a strong authentication protocol. But, there

are times when using PAP can be justified.

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Configuring PPP with Authentication Unlike PAP, which only authenticates once, CHAP conducts

periodic challenges to make sure that the remote node still has a valid password value.

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Configuring PPP with Authentication Configuring PAP

172.25.3.0/24Serial .1/S0.2/S0

DCEDTE

hostname SantaCruzusername HQ password HQpass

interface Serial0 ip address 172.25.3.2 255.255.255.0 encapsulation ppp ppp authentication pap ppp pap sent-username SantaCruz password SantaCruzpass

hostname HQusername SantaCruz password SantaCruzpass

interface Serial0 ip address 172.25.3.1 255.255.255.0 encapsulation ppp ppp authentication pap ppp pap sent-username HQ password HQpass

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Configuring PPP with Authentication Configuring PAP

172.25.3.0/24Serial .1/S0.2/S0

DCEDTE

hostname SantaCruzusername HQ password HQpass

interface Serial0 ip address 172.25.3.2 255.255.255.0 encapsulation ppp ppp authentication pap ppp pap sent-username SantaCruz password SantaCruzpass

hostname HQusername SantaCruz password SantaCruzpass

interface Serial0 ip address 172.25.3.1 255.255.255.0 encapsulation ppp ppp authentication pap ppp pap sent-username HQ password HQpass

Notes: sent-username and password must match remote username and password. Passwords are case-sensitive, but usernames are not. Hostnames are not involved.

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Configuring PPP with Authentication Configuring CHAP

172.25.3.0/24Serial .1/S0.2/S0

DCEDTE

hostname SantaCruzusername HQ password boardwalk

interface Serial0 ip address 172.25.3.2 255.255.255.0 encapsulation ppp ppp authentication chap

hostname HQusername SantaCruz password boardwalk

interface Serial0 ip address 172.25.3.1 255.255.255.0 encapsulation ppp ppp authentication chap

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Configuring PPP with Authentication Explain how to configure a PPP connection with

authentication

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Configuring PPP with Authentication Explain the output of the debug ppp authentication

command

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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

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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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