Cisco CCNA Certification Exam Practice Questions:

DTE/DCE Cables And Directly Connected Serial Interfaces

By Chris Bryant, CCIE #12933

DTE/DCE cables and directly connected Serial interfaces are both important topics on today's CCNA and CCENT exams, and here are some practice exam questions to help you sharpen your skills!

Answers are at the bottom of the page, and when you're done with this practice exam, head over to my CCNA / CCNP certification tutorial page - over 250 practice exams, training videos, and tutorials are waiting for you there!

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.

Here are the answers!

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".

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com