Cisco CCNA Certification Exam Practice Questions:

ISDN, DTE/DCE Cables, Directly Connected Serial Interfaces

By Chris Bryant, CCIE #12933

To help you along in your CCNA certification exam studies, here are ten free questions dealing with ISDN, clockrates, DTE/DCE cables, and more. Enjoy! Answers are posted at the bottom of the page. Enjoy!

1. R1 and R3 are connected via a DTE/DCE cable, both routers using interface serial1. What additional command is needed to make the line protocol come up?  

A. On the DTE, configure "clockrate 56000".

B. On the DCE, configure "clockrate 56000".

C. On the DTE, configure "clockrate 56".

D. On the DCE, configure "clockrate 56".

2. R1 and R3 are directly connected at their respective Serial1 interfaces via a DTE/DCE cable. What command will tell you whether the interface is connected to the DTE or DCE end of the cable?  

A. show dte dce serial 1

B. debug interface serial 1

C. show controller serial 1

D. show cable serial 1

3. The default encapsulation type for a Cisco serial or BRI interface is ____________.

4. You have two routers directly connected with a DTE/DCE cable at their respective serial1 interfaces. R1 is running the default for a serial interface, and R3 is running PPP. Which of the following statements is true?  

A. R1 is running PPP.

B. The physical interfaces on these routers will be up but the line protocol will be down.

C. R1 is running HDLC.

D. R1 is running frame relay.

E. The physical interfaces will show as "down".

F. The different encapsulations do not pose a connectivity problem.

5. Identify the true statements regarding PAP.  

A. PAP is an HDLC authentication option.

B. PAP is a PPP authentication option.

C. PAP sends the username and password in clear text.

D. PAP encrypts the username but not the password.

E. PAP requres the use of the "ppp pap sent-username" interface-level command.

F. PAP allows the routers to use different passwords.

G. PAP requires the routers to use the same passwords.

6. Identify the true statements regarding CHAP.  

A. CHAP is an HDLC authentication protocol.

B. CHAP is a PPP authentication protocol.

C. CHAP requires the passwords to be the same on each router.

D. CHAP allows the passwords to be different on each router.

E. CHAP utilizes a three-way handshake.

F. The CHAP process can be seen with "debug ppp chap".

G. CHAP requires the use of the "ppp chap sent-username" command.

H. CHAP does not use the "ppp chap sent-username" command.

7. Which of the following technologies is generally used as a backup strategy?  

A. ATM

B. Frame Relay

C. PPP

D. ISDN

8. Which of the following is true of ISDN BRI?

A. The d-channel(s) set up the call.

B. The b-channel(s) set up the call.

C. There are three channels, one b-channel and two d-channels.

D. There are three channels, one d-channel and two b-channels.

E. The bandwidth is the same for all three channels.

F. The bandwidth differs between the three channels.

G. The d-channel(s) carry the data, which can be voice or video.

H. The b-channel(s) carry the data, which can be voice or video.

9. Identify the differences between PRI (US), PRI (EU), and BRI.  

A. The bandwidth is the same for d-channels in PRI as it is in BRI.

B. The bandwidth is the same for b-channels in PRI as it is in BRI.

C. Both have the same number of b-channels.

D. Both have the same number of d-channels.

E. These formats have a different number of b-channels.

F. These formats have a different number of d-channels.

10. R1 cannot call R2. You run show isdn status and get the output shown below. What is the issue?

R1#show isdn status

Global ISDN Switchtype = basic-ni

ISDN BRI0 interface

dsl 0, interface ISDN Switchtype = basic-ni

Layer 1 Status: ACTIVE

Layer 2 Status:

TEI = 66, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED

TEI 66, ces = 1, state = 8(established)

spid1 configured, no LDN, spid1 NOT sent, spid1 NOT valid

TEI Not Assigned, ces = 2, state = 1(terminal down)

spid2 configured, no LDN, spid2 NOT sent, spid2 NOT valid

Layer 3 Status:

0 Active Layer 3 Call(s)

A. The spids were incorrectly entered.

B. An ISDN phone call must be active for the spids to take effect.

C. The spids need to be sent to the telco with the "isdn spid send" command.

D. Only one TEI has been assigned; there is an issue with the telco.

And while you're here... join the thousands worldwide who have put the world's most powerful CCNA certification exam study tool to work for them - including my exclusive CCNA Ultimate Study Guide, personal support via my exclusive Cisco forum, training on real Cisco routers and switches, the world's only Binary Math and Subnetting ebook written just for the exam, and more.

It's The Ultimate CCNA Study Package, and it's only available from The Bryant Advantage.

Click that link now to find out more and download this incredible CCNA certification test prep package immediately after your purchase!

Here are the answers to these Cisco CCNA certification exam practice questions!

1 - D. This command needs to be configured on the DCE serial interface, and the value is entered in units of bits per second.

IOS Help is a great help in finding out what unit is used with a given Cisco command. Be careful - some commands use bits per second, some use kilobits per second; some use seconds, others use minutes. Get in the habit of using IOS Help whenever possible.

2 - C. "show controller serial 1" will give you this information. When you run this command, you will get quite a bit of information, but don't worry - the DTE / DCE information is right at the top of the output, in the second line.

3. The default encapsulation for a BRI or Serial interface on a Cisco router is HDLC (High Data-Link Control).

4 - B, C. The default encapsulation for a serial interface is HDLC. An encapsulation mismatch will result in the physical interface remaining up, but the line protocol will come down and stay there until this mismatch is resolved.

5 - B, C, E, F. PAP requires PPP to be configured. PAP will send the username and password in clear text, although the passwords do not have to be the same. A common error with PAP configurations is forgetting to configure the "ppp pap sent-username" command on the BRI interface. This command is required.

6 - B, C, E, H. CHAP does require the password to be the same on both routers, since this password will be hashed via the use of a random number. It is that hash result and random number that is passed over the line - CHAP doesn't send the password and username in clear text.

When the hash result and random number get to the other side of the WAN, the hash is run again - and the hash result must be the password configured on that router for authentication to succeed.

CHAP requires PPP, but does not use any interface-level commands to send the password. There is no such command as "ppp chap sent-username".CHAP does use a three-way handshake, which can be seen with "debug ppp authentication".

7 - D. ISDN is typically used as a backup rather than the primary mode of data transport.

8 - A, D, F, H. There are three channels: one d-channel and two b-channels. The d-channel is responsible for setting up the call. The b-channels carry the load, which can be data, video, or voice. The bandwidth between the channels does differ in BRI. The d-channel has a 16 kbps bandwidth limitation while the b-channels each have 64 kbps available.

9 - B, D, E. BRI has 2 b-channels, PRI (US) has 23, and PRI (EU) has 30. All three formats have one b-channel. The capacity for every b-channel in all of these formats is 64 kbps. The capacity for the d-channel in BRI is 16 kbps, but in both PRI formats, it is 64 kbps.

10. The most likely problem is that the spids were entered incorrectly. They're generally a string of 10 numbers, so it's easy to type them in incorrectly. Real world hint: If the spids look good in your configuration but you continue to get this message, simply reset the interface by shutting it down and reopening it.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com