ICND2 – PPP Questions

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Question 1

[am4show have=’p2;’]Which feature does PPP use to encapsulate multiple protocols?

A. NCP
B. LCP
C. IPCP
D. IPXP

 

Answer: A[/am4show]

Question 2

[am4show have=’p2;’]Which two statements about using the CHAP authentication mechanism in a PPP link are true? (Choose two)

A. CHAP uses a two-way handshake.
B. CHAP uses a three-way handshake.
C. CHAP authentication periodically occurs after link establishment.
D. CHAP authentication passwords are sent in plaintext.
E. CHAP authentication is performed only upon link establishment.
F. CHAP has no protection from playback attacks.

 

Answer: B C[/am4show]

Explanation

Point-to-Point Protocol (PPP) can use either Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) for authentication. CHAP is used upon initial link establishment and periodically to make sure that the router is still communicating with the same host. CHAP passwords arc exchanged as message digest algorithm 5 (MD5) hash values.

The three-way handshake steps are as follows:

Challenge: The authenticator generates a frame called a Challenge and sends it to the initiator. This frame contains a simple text message (sometimes called the challenge text). The message has no inherent special meaning so it doesn’t matter if anyone intercepts it. The important thing is that after receipt of the Challenge both devices have the same challenge message.

Response: The initiator uses its password (or some other shared “secret” that the authenticators also knows) to encrypt the challenge text. It then sends the encrypted challenge text as a Response back to the authenticator.

Success or Failure: The authenticator performs the same encryption on the challenge text that the initiator did. If the authenticator gets the same result that the initiator sent it in the Response, the authenticator knows that the initiator had the right password when it did its encryption, so the authenticator sends back a Success message. Otherwise, it sends a Failure message.

(Reference: CCNA Quick Reference Sheets)

Question 3

[am4show have=’p2;’]What is the purpose of LCP?

A. to perform authentication
B. to negotiate control options
C. to encapsulate multiple protocols
D. to specify asynchronous versus synchronous

 

Answer: B[/am4show]

Question 4

[am4show have=’p2;’]Which command is used to enable CHAP authentication with PAP as the fallback method on a serial interface?

A. (config-if)# authentication ppp chap fallback ppp
B. (config-if)# authentication ppp chap pap
C. (config-if)# ppp authentication chap pap
D. (config-if)# ppp authentication chap fallback ppp

 

Answer: C[/am4show]

Explanation

The command “ppp authentication chap pap” command indicates the CHAP authentication is used first. If it fails or is rejected by other side then uses PAP instead. If you want to use PAP first (then CHAP) you can use the “ppp authentication pap chap” command.

Question 5

[am4show have=’p2;’]What are three reasons that an organization with multiple branch offices and roaming users might implement a Cisco VPN solution instead of point-to-point WAN links? (Choose three)

A. reduced cost
B. better throughput
C. broadband incompatibility
D. increased security
E. scalability
F. reduced latency

 

Answer: A D E[/am4show]

Question 6

[am4show have=’p2;’]Which Layer 2 protocol encapsulation type supports synchronous and asynchronous circuits and has built-in security mechanisms?

A. HDLC
B. PPP
C. X.25
D. Frame Relay

 

Answer: B[/am4show]

Explanation

PPP supports both synchronous (like analog phone lines) and asynchronous circuits (such as ISDN or digital links). With synchronous circuits we need to use clock rate.

Note: Serial links can be synchronous or asynchronous. Asynchronous connections used to be only available on low-speed (<2MB) serial interfaces, but now, there are the new HWICs (High-Speed WAN Interface Cards) which also support asynchronous mode. To learn more about them please visit http://www.cisco.com/en/US/prod/collateral/modules/ps5949/ps6182/prod_qas0900aecd80274424.html.

Question 7

[am4show have=’p2;’]Which PPP subprotocol negotiates authentication options?

A. NCP
B. ISDN
C. SUP
D. LCP
E. DLCI

 

Answer: D[/am4show]

Explanation

Link Control Protocol (LCP) is a subprotocol within the Point-to-Point Protocol protocol suite that is responsible for link management. During establishment of a PPP communication session, LCP establishes the link, configures PPP options, and tests the quality of the line connection between the PPP client and PPP server. LCP automatically handles encapsulation format options and varies packet sizes over PPP communication links.

LCP also negotiates the type of authentication protocol used to establish the PPP session. Different authentication protocols are supported for satisfying the security needs of different environments.

Other subprotocol within PPP is Network Control Protocol (NCP), which is used to allow multiple Network layer protocols (routed protocols) to be used on a point-to-point connection.

Question 8

[am4show have=’p2;’]At which layer of the OSI model does PPP perform?

A. Layer 2
B. Layer 3
C. Layer 4
D. Layer 5

 

Answer: A[/am4show]

Explanation

Layer 2 includes the popular WAN standards, such as the Point-to-Point Protocol (PPP), High-Level Data-Link Control (HDLC) and Frame Relay protocols.