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316 Chapter 9: Point-to-Point Leased Line Implementation

Foundation Summary

The “Foundation Summary” section lists the most important facts from the chapter. Although this section does not list everything that will be on the exam, a well-prepared CCNA candidate should at a minimum know all the details in each Foundation Summary before taking the exam.

Table 9-7 gives the basic definitions of the three types of WAN services.

Table 9-7 Definitions of Leased Line, Circuit Switching, and Packet Switching

Term

Description

 

 

Leased line

A dedicated, always-on circuit between two endpoints. The service

 

provider just passes a constant rate bit stream; it does not interpret

 

or make decisions based on the bits sent over the circuit. Generally

 

is more expensive than packet switching today.

 

 

Circuit switching/dial

Provides dedicated bandwidth between two points, but only for the

 

duration of the call. Typically used as a cheaper alternative to leased

 

lines, particularly when connectivity is not needed all the time. Also

 

is useful for backup when a leased line or packet-switched service

 

fails.

 

 

Packet switching

Provides virtual circuits between pairs of sites, with contracted

 

traffic rates for each VC. Each site’s physical connectivity consists of

 

a leased line from the site to a device in the provider’s network.

 

Generally cheaper than leased lines.

 

 

The physical connection for leased lines includes a CSU/DSU on each end of the link, as shown in Figure 9-5.

Figure 9-5 Physical Components of a Point-to-Point Serial Link

Short Cables (Usually Less than 50 Feet)

Long Cables (Can Be Several Miles Long)

 

 

 

 

 

 

 

 

 

 

 

 

TELCO

 

 

 

 

 

 

 

 

CSU

 

 

 

 

 

 

 

 

 

 

R1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CPE

 

 

 

 

 

 

WAN Switch

 

 

WAN Switch

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CO

 

CO

 

 

 

 

 

 

 

 

 

demarc

CSU/DSU

CSU R2

CPE

demarc

Foundation Summary 317

Table 9-8 lists the important comparison points between HDLC and PPP.

Table 9-8

Point-to-Point Data-Link Protocol Attributes

 

 

 

 

 

 

 

 

 

Architected Type

 

 

Protocol

Error Correction

Field

Other Attributes

 

 

 

 

 

 

HDLC

No

No

HDLC serves as Cisco’s default on

 

 

 

 

serial links. Cisco uses a Proprietary

 

 

 

 

Type field to support multiprotocol

 

 

 

 

traffic. Supports synchronous links

 

 

 

 

only.

 

 

 

 

 

 

PPP

Supported but not

Yes

PPP was meant for multiprotocol

 

 

enabled by default

 

interoperability from its inception,

 

 

 

 

unlike HDLC. PPP also supports

 

 

 

 

asynchronous communication.

 

 

 

 

 

Figure 9-6 outlines the different PAP and CHAP flows.

Figure 9-6 PAP and CHAP Messages

 

 

PAP

 

Waiting

 

 

Dialing

for Dial

I Am Barney

Router

 

 

 

 

 

 

 

Password = Betty

 

Fred

 

Ack.

Barney

 

 

 

 

 

 

 

 

CHAP

 

 

Waiting

 

Challenge!

 

Dialing

for Dial

 

 

Router

 

 

 

 

 

 

 

I Am #$%#$@

 

 

Fred

 

 

 

Barney

 

Accepted

 

 

 

 

 

 

 

 

 

 

318 Chapter 9: Point-to-Point Leased Line Implementation

Q&A

As mentioned in the Introduction, you have two choices for review questions. The following questions give you a bigger challenge than the exam because they are open-ended. By reviewing with this more-difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of the topics covered in this chapter. The answers to these questions are found in Appendix A.

For more practice with exam-like question formats, including multiple-choice questions and those using a router simulator, use the exam engine on the CD.

1.Define the terms DCE and DTE in the context of the physical layer and a point-to-point serial link.

2.Identify the command used to set the clock rate on an interface in which a DCE cable has been inserted. Compare and contrast the two styles of the command that may be entered in configuration mode versus which style shows up in the configuration.

3.Name one WAN data-link protocol used on point-to-point leased lines that defines a method of announcing the interface’s IP addresses to other devices attached to the WAN.

4.Can PPP dynamically assign IP addresses? If so, is this feature always enabled?

5.Create a configuration to enable PPP on serial 0 for IP. Make up IP Layer 3 addresses as needed.

6.Define the terms PAP and CHAP. Which one sends passwords in clear-text format?

7.CHAP configuration uses names and passwords. Given Routers A and B, describe what names and passwords must match in the respective CHAP configurations.

8.What field has Cisco added to the HDLC header, making it proprietary?

9.List the types of packet switching mentioned in this chapter. Identify the one for which all traffic is segmented into smaller pieces.

10.Two-wire leased circuits are seldom used today, and certainly not between two routers. What problem was solved with the advent of four-wire leased circuits versus two-wire?

11.Define “synchronous” in relation to leased lines.

12.Create a CHAP configuration between two routers. Make up specific details as needed.

This chapter covers the following subjects:

ISDN Protocols and Design

ISDN Configuration and Dial-on- Demand Routing