C H A P T E R10

ISDN and Dial-on-Demand

Routing

Most of the time, sites that use routers need some sort of permanent, always-on WAN connectivity to other sites in the network. However, that permanent connectivity might fail, so these routers might want to use some sort of dial (circuit-switched) technology to be able to send and receive packets while the failed leased line or packet-switched service is being repaired. Also, for sites that truly need only occasional WAN connectivity, dialed circuits, for which a service provider charges a small fee per call and per minute, might actually be cheaper than using a leased line or packet-switched service. So, although they aren’t the typical choice for WAN connectivity for everyday use, dialed links are still popular.

Of all the circuit-switched “dial” options that have emerged over the years, ISDN is probably the most popular choice for connectivity between routers. ISDN uses digital signals, which allows for faster speeds than analog lines. ISDN has grown to be commonly available in many countries and cities, so it is a viable option for many companies worldwide.

This chapter covers the basics of ISDN technology, along with the related router configuration. In particular, the Cisco IOS software feature called dial-on-demand routing (DDR), which allows you to essentially define when the dialed connection should be brought up and taken down, is covered in detail.

“Do I Know This Already?” Quiz

The purpose of the “Do I Know This Already?” quiz is to help you decide if you need to read the entire chapter. If you intend to read the entire chapter, you do not necessarily need to answer these questions now.

The ten-question quiz, derived from the major sections in the “Foundation Topics” section, helps you determine how to spend your limited study time.

322 Chapter 10: ISDN and Dial-on-Demand Routing

Table 10-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.

Table 10-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you don’t know the answer to a question or you’re only partially sure of the answer, you should mark this question as wrong for purposes of the self-assessment. Giving yourself credit for an answer you guess correctly skews your self-assessment results and might give you a false sense of security.

1.How fast do the D channels run on a BRI and PRI, respectively?

a.8 kbps and 8 kbps

b.8 kbps and 16 kbps

c.8 kbps and 64 kbps

d.16 kbps and 16 kbps

e.16 kbps and 64 kbps

f.64 kbps and 64 kbps

g.None of the above

2.Which of the following protocols defines call setup signaling for ISDN?

a.I.411

b.I.319

c.Q.911

d.Q.921

e.Q.931

f.Q.941

g.None of the above

“Do I Know This Already?” Quiz 323

3.Which of the following reference points are used by ISDN BRI interfaces in a Cisco router?

a.R

b.R/S

c.S/T

d.U

e.V

4.Which of the following PRI function groups is appropriate for use with a PC with a simple serial interface?

a.TA

b.NT1

c.NT2

d.SBus

e.None of the above

5.Which of the following encoding formats could be used on a PRI in North America?

a.AMI

b.B8ZS

c.SF

d.ESF

e.10B8N

f.None of the above

324Chapter 10: ISDN and Dial-on-Demand Routing

6.Assume that a router has an IP ACL numbered 109 that permits traffic going to IP address 10.1.1.1. Which of the following commands correctly refers to the ACL, causing traffic sent to IP address 10.1.1.1 to be considered “interesting”?

a.dialer acl 109

b.dialer list 109

c.dialer-list 1 protocol ip 10.1.1.1

d.dialer list 1 protocol ip 10.1.1.1

e.dialer-list 1 protocol ip list 109

f.dialer list 1 protocol ip list 109

g.dialer-list 1 list 109

h.dialer list 1 list 109

i.None of the above

7.Which of the following could be necessary in a configuration that uses DDR and ISDN BRI interfaces?

a.Switch type

b.Encoding

c.Framing

d.SPIDs

e.Identification of DS0 channels used

f.Polarity

g.None of the above

8.Which of the following could be necessary in a configuration that uses DDR and ISDN PRI interfaces in North America?

a.Switch type

b.Encoding

c.Framing

d.SPIDs

e.Identification of DS0 channels used

f.Polarity

g.None of the above

“Do I Know This Already?” Quiz 325

9.When you configure DDR using the style of DDR configuration called dialer profiles, which of the following commands are useful on the dialer interface?

a.dialer-group

b.dialer-list

c.encapsulation ppp

d.All of the above

e.None of the above

10.Which of the following interface subcommands enables multilink PPP?

a.ppp multilink

b.encapsulation ppp multilink

c.enable mlp

d.mlp on

e.None of the above

The answers to the “Do I Know This Already?” quiz appear in Appendix A. The suggested choices for your next step are as follows:

■8 or less overall score—Read the entire chapter. This includes the “Foundation Topics,” “Foundation Summary,” and “Q&A” sections.

■9 or 10 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the “Q&A” section. Otherwise, move to the next chapter.