Chapter 19: Quiz – Understanding Wireless Roaming and Location Services (Answers) CCNPv8 ENCOR

25. What is a reason to specify lightweight APs and a WLAN controller as the major elements in a WLAN design?

  • Lightweight APs can be controlled wirelessly.
  • The design provides a user-friendly standalone operation.
  • The design simplifies the deployment of roaming features.
  • The design uses a single end-to-end VLAN to provide wireless roaming features.

Explanation: A lightweight AP and a wireless LAN controller are used in a corporate wireless design. Multiple APs are controlled by a wireless LAN controller that can support wireless VoIP, wireless roaming, network management, and AP upgrades.

26. What is the minimum number of APs to be used to achieve an accurate location of wireless clients based on RSS values?

  • 2
  • 3
  • 4
  • 5

Explanation: To locate a device more accurately, an AP can use the received signal strength (RSS) of a client device as a measure of the distance between the two. However, if the distance is measured from a single AP only, it is difficult to determine where the client is situated in relation to the AP. A better solution is to obtain the same measurement from three or more APs, then correlate the results and determine where they intersect.

27. Which technique is used by Cisco WLCs to minimize the time required on key exchanges during roaming by using an amendment to WLAN standards to address fast roaming or fast BSS transition?

  • key caching
  • 802.11r support
  • intercontroller roaming
  • Cisco Centralized Key Management (CCKM)

Explanation: Cisco controllers offer three techniques to minimize the time and effort spent on key exchanges during roams:

  • Cisco Centralized Key Management (CCKM): One controller maintains a database of clients and keys on behalf of bound APs and provides them to other controllers and bound APs as needed during client roams.
  • Key caching: Each client maintains a list of keys used with prior AP associations and presents the keys as it roams.
  • 802.11r: This 802.11 amendment addresses fast roaming or fast BSS transition. A client can cache a portion of the key of the authentication server and present that to future APs as it roams.

28. A company deploys a wireless network on campus. The network analysts use multiple APs to locate wireless clients based on the RSS values received by the APs. What is a reason that the method may not provide accurate location information all the time?

  • The method assumes that the APs and client devices are Cisco products.
  • The method assumes that the client devices are using intercontroller roaming.
  • The method assumes that the client devices are using intracontroller roaming.
  • The method assumes that the APs and client devices are in open free space with no physical obstacles.

Explanation: The method to obtain the RSS values from three or more APs and correlate the results and determine where they intersect is based on the assumption that the APs and client devices are located in open free space, with nothing but free space path loss to attenuate the RF signal. In a normal environment, the APs and clients exist in buildings where physical objects, such as walls, doors, windows, furniture, cubicles, and shelving, also exist and get in the way of the RF signals. Usually the signals can pass through various materials but get attenuated along the way. That further complicates determining device location accurately.

29. What is a difficulty in locating a wireless client according to the location of the AP to which the client is currently joined?

  • The client may roam away from the AP.
  • The client may fail to associate with the AP.
  • The client may fail to authenticate with the AP.
  • The client location target may not be granular enough.

Explanation: Before each wireless client can use the network, it must first be authenticated and associated by an AP. Thus, a client can be located according to the AP to which it is currently joined. However, that may not be granular enough for every use case because one AP might cover a large area.

30. A company is deploying a wireless network on campus using the lightweight AP topology. Which two types of roaming could occur? (Choose two.)

  • end user roaming
  • enterprise roaming
  • autonomous roaming
  • intercontroller roaming
  • intracontroller roaming

Explanation: In a lightweight AP topology, APs are bound to a wireless LAN controller. The WLC performs the management functions such as roaming management. When a wireless client is roaming among APs that are bound to the same WLC controller, the roam occurs entirely within the controller. This is known as intracontroller roaming. Intercontroller roaming occurs when a client is roaming among APs that are bound to different WLC controllers.

31. When wireless clients move from one AP to another, which device determines when a roaming session starts?

  • WLC
  • wireless client
  • AP to which the client will move
  • switch to which the current AP is connected

Explanation: A wireless client continuously evaluates the quality of the wireless connection, whether it is moving around or not. If the signal quality degrades, the client will begin looking for a different AP that can offer a better signal.

32. Which two Cisco management platforms can integrate with APs and WLCs along with location servers to provide real-time location services for a wireless network? (Choose two.)

  • DNA Space
  • DNA Center
  • Prime Infrastructure
  • Mobility Services Engine
  • Connected Mobile Experiences

Explanation: Cisco APs and WLCs can integrate with management platforms like Cisco Prime Infrastructure or DNA Center, along with location servers like Cisco Mobility Services Engine (MSE), Cisco Connected Mobile Experiences (CMX), or Cisco DNA Spaces to gather location information in real time and present that information in a relevant way.

33. What is the purpose of a wireless client sending an association request to an AP?

  • to find the SSID of the AP
  • to form a new association with the AP
  • to roam from current AP to another AP
  • to respond to the beacon signal sent by the AP

Explanation: When a client wants to join the BSS of an AP, it actively scans channels and sends probe requests to discover candidate APs. The client then selects one and tries to associate with it. A wireless client can send association request and reassociation request frames to the AP selected. Association requests are used to form a new association, whereas reassociation requests are used to roam from one AP to another, preserving the original association status of the client.

34. What is a characteristic of mobility groups?

  • Mobility groups are organized in a tree structure.
  • Members of a mobility group can be dynamically joined.
  • A mobility group consists of one centralized WLC and several connected APs.
  • A mobility group is a group of WLCs in a network with the same mobility group name.

Explanation: Cisco controllers can be organized into mobility groups to facilitate intercontroller roaming. A mobility group is a group of WLCs in a network with the same mobility group name. A mobility group is configured manually.

35. A company deploys a lightweight AP topology on campus. Which scenario is an example of Layer 2 intercontroller roaming?

  • A client roams from one AP to another AP that is in autonomous mode.
  • A client roams from one AP to another AP that is bound to the same WLC.
  • A client roams from one AP to another AP that is configured with a private IP address subnet.
  • A client roams from one AP to another AP that is configured with the same VLAN and IP subnet.

Explanation: In intercontroller roaming, a client roams from one AP to another AP that is bound to a different WLC. If both APs are configured with the same VLAN and IP address subnet, the client has made a Layer 2 intercontroller roam and it stays on the same VLAN and subnet.

36. Which type of wireless client roaming is associated with an anchor and a foreign controller?

  • Layer 2 roaming
  • Layer 3 roaming
  • intracontroller roaming
  • user-to-internet roaming

Explanation: A Layer 3 intercontroller roam consists of an extra tunnel that is built between the original controller that the client was associated with (called the anchor controller) and the controller it has roamed to (called the foreign controller).

“Do I Know This Already?” Quiz Answers:

1. When a client moves its association from one autonomous AP to another, it is actually leaving and joining which one of the following?

  • SSID
  • BSS
  • ESS
  • DS

Explanation: The client must associate with a BSS offered by an AP.

2. Which one of the following makes the decision for a device to roam from one AP to another?

  • The client device
  • The original AP
  • The candidate AP
  • The wireless LAN controller

Explanation: The client device is in complete control of the roaming decision, based on its own roaming algorithm. It uses active scanning and probing to discover other candidate APs that it might roam to.

3. Ten lightweight APs are joined to a wireless LAN controller. If a client roams from one of the APs to another, which one of the following correctly describes the roam?

  • Autonomous roaming
  • Intercontroller roaming
  • Intracontroller roaming
  • Indirect roaming

Explanation: Because a single controller is involved, the roam occurs in an intracontroller fashion. Even though the client thinks it is associating with APs, the associations actually occur at the controller, thanks to the split-MAC architecture.

4. Which of the following provides the most efficient means for roaming, as measured by the time to complete the roam?

  • Layer 2 intercontroller roaming
  • Layer 3 intercontroller roaming
  • Intracontroller roaming
  • All of the above; they all take equal amounts of time.

Explanation: Intracontroller roaming is the most efficient because the reassociation and client authentication occur within a single controller.

5. Which of the following is used to cache authentication key information to make roaming more efficient?

  • PGP
  • CCNA
  • CCKM
  • EoIP

Explanation: Cisco Centralized Key Management (CCKM) is used to cache key information between a client and an AP. The cached information is then used as a quick check when a client roams to a different AP.

6. In a Layer 2 roam, what mechanism is used to tunnel client data between the two controllers?

  • GRE tunnel
  • EoIP tunnel
  • CAPWAP tunnel
  • None of these answers

Explanation: In a Layer 2 roam, the client’s IP subnet does not change as it moves between controllers. Therefore, there is no need to tunnel the client data between the controllers; instead, the client simply gets handed off to the new controller.

7. A client roams from controller A to controller B. If it undergoes a Layer 3 roam, which one of the following best describes the role of controller A?

  • Foreign controller
  • Host controller
  • Master controller
  • Anchor controller

Explanation: The anchor controller, where the client starts, maintains the client’s state and builds a tunnel to the foreign controller, to which the client has now roamed.

8. A network consists of four controllers: A, B, C, and D. Mobility group 1 consists of controllers A and B, while mobility group 2 consists of controllers C and D. Which one of the following answers describes what happens when a client tries to roam between controllers B and C?

  • Roaming is seamless and effi cient.
  • Roaming is not possible.
  • Roaming is possible, but CCKM and key caching do not work.
  • Only Layer 3 roaming is possible.

Explanation: Controllers A and B are listed in each other’s mobility list, so they are known to each other. However, they are configured with different mobility group names. Clients may roam between the two controllers, but CCKM and PKC information will not be exchanged.

9. Which of the following parameters is useful for computing a client device’s location with respect to an AP?

  • BSS
  • GPS
  • RSS
  • Channel

Explanation: The client’s received signal strength (RSS) can be used to calculate an approximate distance from the AP based on the free space path loss attenuation.

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