EWAN v4 Chapter 2 Check Your Understanding: PPP

1. Match each PPP establishment step with its appropriate sequence number:

Step 1 Send link-establishment frames to negotiate options such as MTU size, compression, and authentication.
Step 2 Send configuration-acknowledgment frames.
Step 3 Test link quality (optional).
Step 4 Negotiate Layer 3 protocol options.
Step 5 NCP reaches Open state.

2. Which output from the show interfaces s0/0/0 command indicates that the far end of a point-to-point link has a different encapsulation set than the local router?

  • serial 0/0/0 is down, line protocol is down
  • serial 0/0/0 is up, line protocol is down
  • serial 0/0/0 is up, line protocol is up (looped)
  • serial 0/0/0 is up, line protocol is down (disabled)
  • serial 0/0/0 is administratively down, line protocol is down

Explanation: This is the state of the interface when the local or remote router is misconfigured with a different Layer 2 encapsulation. In our labs, a missing clock rate would also cause this message to appear.

3. What is the default encapsulation for serial interfaces on a Cisco router?

  • HDLC
  • PPP
  • Frame Relay
  • X.25

Explanation: Cisco HDLC is the default encapsulation method used by Cisco devices on synchronous serial lines.

4. What is the function of the Protocol field in a PPP frame?

  • It identifies the application layer protocol that will process the frame.
  • It identifies the transport layer protocol that will process the frame.
  • It identifies the data link layer protocol encapsulation in the frame’s Date field.
  • It identifies the network layer protocol encapsulated in the frame’s Data field.

Explanation: The protocol field in a PPP frame indicates the type of Layer 3 information is carried in the frame’s Data field.

5. Match each description with its corresponding term:

Error control
Authentication protocols
Allows load balancing
Compression protocols

A. Stacker/predictor
B. Magic number
C. Multilink
E. Call in

Error control Magic number
Authentication protocols CHAP/PAP
Allows load balancing Multilink
Compression protocols Stacker/predictor

6. Which of the following statements describe the function of statistical time-division multiplexing (STDM)? (Choose three.)

  • Multiple data streams share one common channel.
  • Bit interleaving controls the timing mechanism that places data on the channel.
  • Time slots are used on a first-come, first-served basis.
  • STDM was developed to overcome the inefficiency caused by time slots still being allocated even when the channel has no data to transmit.
  • Sources of data alternate during transmission and are reconstructed at the receiving end.
  • Priority can be dedicated to one data source.

7. Which of the following describes the serial connection between two routers using the High-level Data Link Control (HDLC) protocol?

  • Synchronous or asynchronous bit-oriented transmissions using a universal frame format
  • Synchronous bit-oriented transmissions using a frame format that allows flow control and error detection
  • Asynchronous bit-oriented transmissions using a frame format derived from the Synchronous Data Link Control (SDLC) protocol
  • Asynchronous bit-oriented transmissions using a V.35 DTE/DCE interface

Explanation: HDLC is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). HDLC was developed from the Synchronous Data Link Control (SDLC) standard proposed in the 1970s. HDLC provides both connection-oriented and connectionless service.

8. If an authentication protocol is configured for PPP operation, when is the client or user workstation authenticated?

  • Before link establishment
  • During the link establishment phase
  • Before the network layer protocol configuration begins
  • After the network layer protocol configuration has ended

Explanation: Authentication takes place after link establishment and before any network layer protocol configuration.

9. Why are Network Control Protocols used in PPP?

  • To establish and terminate data links
  • To provide authentication capabilities to PPP
  • To manage network congestion and to allow quality testing of the link
  • To allow multiple Layer 3 protocols to operate over the same physical link

Explanation: PPP uses the Network Control Protocol (NCP) process to negotiate the Layer 3 protocols to operate over the link. IPCP is an example of an NCP for IP.

10. Which statement describes the PAP authentication protocol?

  • It sends encrypted passwords by default.
  • It uses a two-way handshake to establish identity.
  • It protects against repeated trial-and-error attacks.
  • It requires the same username to be configured on every router.

Explanation: PAP uses a two-way handshake, whereas CHAP uses a three-way handshake. The other three possible answers are features of CHAP.

11. A technician testing the functionality of a recently installed router is unable to ping the serial interface of a remote router. The technician executes the show interfaces serial 0/0/0 command on the local router and sees the following line in the router:

Serial0/0/0 is down, line protocol is down

What are two possible causes of this command output?

  • The clock rate command is missing.
  • The carrier detect signal is not sensed.
  • Keepalives are not being sent.
  • The interface is disabled due to a high error rate.
  • The interface is shut down.
  • The cabling is faulty or incorrect.

Explanation: This state can be caused by the router’s not sensing a CD signal, which means that the CD is not active. This state can also be caused by faulty or incorrect cabling. Other causes include a WAN carrier service provider problem, which means that the line is down or is not connected to the CSU/DSU, or a hardware failure (CSU/DSU).

12. The network administrator is configuring Router1 to connect to Router2 using threeway handshake authentication. Match each description with the command necessary to configure Router1:

Configure the username and password
Enter interface configuration mode
Specify the encapsulation type
Configure authentication

A. username Router2 password cisco
B. username Router1 password cisco
C. interface serial 0/1/0
D. encapsulation ppp
E. encapsulation hdlc
F. ppp authentication pap
G. ppp authentication chap

Configure the username and password username Router2 password cisco
Enter interface configuration mode interface serial 0/1/0
Specify the encapsulation type encapsulation ppp
Configure authentication ppp authentication chap

13. What is required to successfully establish a connection between two routers using CHAP authentication?

  • The hostnames of both routers must be the same.
  • The usernames of both routers must be the same.
  • The enable secret passwords configured on both routers must be the same.
  • The password configured with the router’s username must be the same on both routers.
  • The ppp chap sent-username command must be configured the same on both routers.

Explanation: With CHAP authentication, the hostname on one router must match the username the other router has configured. The passwords must also match on both routers.

14. For each characteristic, indicate whether it is associated with PAP or CHAP:

Two-way handshake PAP
Three-way handshake CHAP
Open to trial-and-error attacks PAP
Password sent in clear text PAP
Periodic verification CHAP
Uses a one-way hash function CHAP

15. For each description, indicate whether it is associated with LCP or NCP:

Negotiates link establishment parameters LCP
Negotiates Layer 3 protocol parameters NCP
Maintains/debugs a link LCP
Can negotiate multiple Layer 3 protocols NCP
Terminates a link LCP

16. Describe four of the six types of WAN encapsulation protocols.

  • HDLC: The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices.
  • PPP: Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several network layer protocols, such as IP and IPX. PPP also has built-in security mechanisms such as PAP and CHAP.
  • Serial Line Internet Protocol (SLIP): A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.
  • X.25/Link Access Procedure, Balanced (LAPB): An ITU-T standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data link layer protocol. X.25 is a predecessor to Frame Relay.
  • Frame Relay: Industry-standard, switched, data link layer protocol that handles multiple virtual circuits. Frame Relay is a next-generation protocol after X.25. Frame Relay eliminates some of the time-consuming processes (such as error correction and flow control) employed in X.25.
  • ATM: The international standard for cell relay in which devices send multiple service types (such as voice, video, or data) in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays. ATM takes advantage of high-speed transmission media such as E3, SONET, and T3.

17. Describe the functions of LCP and NCP.

Link Control Protocol (LCP) layer

  • Sits on top of the physical layer and has a role in establishing, configuring, and testing the data-link connection.
  • Establishes the point-to-point link.
  • Provides automatic configuration of the interfaces at each end. This includes handling varying limits on packet size, detecting common misconfiguration errors, terminating the link, and determining when a link is functioning properly or when it is failing.
  • Is also used to negotiate authentication, compression, error detection, multilink, and PPP callback after the link is established.
  • Negotiates and sets up control options on the WAN data link, which are handled by the NCPs.

Network Control Protocol (NCP) layer

  • Includes functional fields containing standardized codes to indicate the network layer protocol that PPP encapsulates.
  • Handles the assignment and management of IP addresses in IPCP.
  • Encapsulates and negotiates options for multiple network layer protocols.

18. Describe the five configurable LCP encapsulation options.

Authentication using PAP or CHAP

  • If all you need is password authentication, configure PAP using the ppp authentication pap command.
  • If you want a challenge handshake, configure CHAP using the ppp authentication chap command (it’s more secure).


  • Increases the effective throughput by reducing the amount of data in the PPP frame that must travel across the link.
  • To configure Stacker, use the compress stac command. To configure Predictor, use the compress predictor command.

Error detection

  • Identifies fault conditions to help ensure a reliable, loop-free data link.
  • Configured using the ppp quality percentage command.


  • Provides load balancing over the router interfaces that PPP uses using the command ppp multilink.

PPP callback

  • Enhances security by making a Cisco router a callback client. It makes the initial call, requests that the other Cisco router configured as a server call it back, and terminates its initial call.
  • The command is ppp callback [accept | request].

19. Refer to the following configurations for Router R1 and Router R3:

hostname R1
username R1 password cisco123
int serial 0/0
ip address
encapsulation ppp
ppp authentication pap
hostname R3
username R1 password cisco
int serial 0/0
ip address
encapsulation ppp
ppp authentication CHAP

Router R1 is unable to connect with Router R3. On the basis of the information presented, which configuration changes on Router R1 would correct the problem?

On Router R1:

  • The username command has two errors. The router name should be R3, and the password should be cisco. Therefore, the correct command is username R3 password cisco.
  • The third error is in the ppp authentication command. It should be ppp authentication chap.

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