CCNA 200-125 Exam: WAN Questions With Answers

  1. Which command can you enter to determine whether serial interface 0/2/0 has been configured using HDLC encapsulation?
    • A. router#show platform
    • B. router#show ip interface s0/2/0
    • C. router#show interfaces Serial 0/2/0*
    • D. router#show ip interface brief

    Show (Hide) Explanation/Reference
    An example of the output of the “show interface serial …” command is shown below. We can see the line “Encapsulation HDLC”.

    CCNA 200-125 Exam: WAN Questions With Answers 1

  2. Which Layer 2 protocol encapsulation type supports synchronous and asynchronous circuits and has built-in security mechanisms?
    • A. X.25
    • B. HDLC
    • C. PPP*
    • D. Frame Relay

    Show (Hide) Explanation/Reference
    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

  3. Which statements about using leased lines for your WAN infrastructure are true?
    • A. Leased lines provide inexpensive WAN access.
    • B. Leased lines with sufficient bandwidth can avoid latency between endpoints.*
    • C. Leased lines require little installation and maintenance expertise.*
    • D. Leased lines provide highly flexible bandwidth scaling.
    • E. Multiple leased lines can share a router interface.
    • F. Leased lines support up to T1 link speeds.

    Show (Hide) Explanation/Reference
    The advantages of leased lines include:

    + Simplicity: Point-to-point communication links require minimal expertise to install and maintain.

    + Quality: Point-to-point communication links usually offer high service quality, if they have adequate bandwidth. The dedicated capacity removes latency or jitter between the endpoints.

    + Availability: Constant availability is essential for some applications, such as e-commerce. Point-to-point communication links provide permanent, dedicated capacity, which is required for VoIP or Video over IP.

    The disadvantages of leased lines include:

    + Cost: Point-to-point links are generally the most expensive type of WAN access. The cost of leased line solutions can become significant when they are used to connect many sites over increasing distances. In addition, each endpoint requires an interface on the router, which increases equipment costs.

    + Limited flexibility: WAN traffic is often variable, and leased lines have a fixed capacity, so that the bandwidth of the line seldom matches the need exactly (therefore answer D is not correct). Any change to the leased line generally requires a site visit by ISP personnel to adjust capacity.

    (Reference: Connecting Networks Companion Guide Book published by Cisco Networking Academy – Page 54)

  4. Two routers named Atlanta and Brevard are connected by their serial interfaces as illustrated, but there is no connectivity between them. The Atlanta router is known to have a correct configuration.
    Given the partial configurations, identify the problem on the Brevard router that is causing the lack of connectivity.

    CCNA 200-125 Exam: WAN Questions With Answers 2

    • A. transmission unit size too large
    • B. no loopback set
    • C. an incorrect subnet mask
    • D. incompatible encapsulation at each end
    • E. an incorrect IP address*
    • F. incompatible bandwidth between routers
  5. Which of the following describes the roles of devices in a WAN? (Choose three)
    • A. A CSU/DSU terminates a digital local loop*
    • B. A modem terminates a digital local loop
    • C. A CSU/DSU terminates an analog local loop
    • D. A modem terminates an analog local loop*
    • E. A router is commonly considered a DTE device*
    • F. A router is commonly considered a DCE device

    Show (Hide) Explanation/Reference
    The idea behind a WAN is to be able to connect two DTE networks together through a DCE network. The network’s DCE device (includes CSU/DSU) provides clocking to the DTE-connected interface (the router’s serial interface).

    CCNA 200-125 Exam: WAN Questions With Answers 3

    A modem modulates outgoing digital signals from a computer or other digital device to analog signals for a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device. A CSU/DSU is used between two digital lines -> A & D are correct but B & C are not correct.

    For more explanation of answer D, in telephony the local loop (also referred to as a subscriber line) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider’s network. Therefore a modem terminates an analog local loop is correct.

    CCNA 200-125 Exam: WAN Questions With Answers 4

  6. Which two pieces of information are provided by the “show controllers serial 0” command? (Choose two)
    • A. the type of cable that is connected to the interface.*
    • B. The uptime of the interface
    • C. the status of the physical layer of the interface*
    • D. the full configuration of the interface
    • E. the interface‟s duplex settings

    Show (Hide) Explanation/Reference
    Below is an example of the output of this command:

    CCNA 200-125 Exam: WAN Questions With Answers 5

    The “show controllers serial …” command tells us about the type of the cable (in the case V.35 DTE cable) and the status of the physical layer of the interface. In above output we learn that there is an cable attached on S0/0 interface. If no cable is found we will see the line “No DTE cable” instead.

  7. Which command is used to know the duplex speed of serial link?
    • A. show line
    • B. show interface*
    • C. show protocol
    • D. show run

    Show (Hide) Explanation/Reference
    Nowadays all serial links are full-duplex (as serial interfaces have separate Rx & Tx pins) so maybe this question wants to ask about how to check the speed of the serial link. The “show interface” command gives us information about this. An example of this command is shown below:

    CCNA 200-125 Exam: WAN Questions With Answers 6

    In this output the speed of S0/0 interface is 1544 Kbits.

  8. Which WAN topology provides a direct connection from each site to all other sites on the network?
    • A. single-homed
    • B. full mesh*
    • C. point-to-point
    • D. hub-and-spoke

    Show (Hide) Explanation/Reference
    Full-mesh is a network topology in which there is a direct link between all pairs of nodes. Below is an example of full-mesh topology.

    CCNA 200-125 Exam: WAN Questions With Answers 7

  9. 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
  10. Which three statements about DWDM are true? (Choose three)
    • A. It allows a single strand of fiber to support bidirectional communications*
    • B. It is used for long-distance and submarine cable systems*
    • C. It can multiplex up to 256 channels on a single fiber
    • D. It supports both the SDH and SONET standards*
    • E. Each channel can carry up to a 1-Gbps signal
    • F. It supports simplex communications over multiple strands of fiber

    Show (Hide) Explanation/Reference
    A newer fiber-optic media development for long-range communications is called dense wavelength-division multiplexing (DWDM). DWDM multiplies the amount of bandwidth that a single strand of fiber can support.

    DWDM circuits are used in all modern submarine communications cable systems and other long-haul circuits.

    Specifically, DWDM:

    Enables bidirectional communications over one strand of fiber -> Answer A is correct
    + Assigns incoming optical signals to specific wavelengths of light (i.e., frequencies)
    + Each channel is capable of carrying a 10-Gbps multiplexed signal -> Answer E is not correct
    + Can multiplex more than 80 different channels of data (i.e., wavelengths) onto a single fiber -> Answer C is not correct
    + Can amplify these wavelengths to boost the signal strength
    Supports SONET and SDH standards


  11. Which PPP subprotocol negotiates authentication options?
    • A. NCP
    • B. ISDN
    • C. SUP
    • D. LCP *
    • E. DLCI

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