Traffic Flow and AP

Here you will find answers to Traffic Flow and AP QuestionsAccording to the IEEE 802.11 standard

Question 1

What two statements about beacon frames used by access points are true? (Choose two)

A. contains SSIDs if this feature is enabled
B. provides vendor proprietary information
C. is another name for an associated request
D. sent in response to a probe frame
E. includes ATIM window information for power save operations

 

Answer: A B

Explanation

Prior to authentication/association, devices must identify any access-points that are within range. Two methods of scanning are available; namely, passive and active. In passive scanning mode, a device monitors each channel for beacon frames that are broadcast by the access-points in the vicinity.

A beacon frame is a management frame in IEEE 802.11-based wireless LANs. It contains all the information about the network. Beacon frames are transmitted periodically to announce the presence of a wireless LAN network and are transmitted by the Access Point (AP). A beacon frame includes timestamp, beacon interval, its capabilities, SSIDs, supported rates,  TIM (Traffic Indication Map) and some configuration and security information to the client devices. Some of them are standard; other are vendor proprietary. Below shows a beacon frame with “Cisco Proprietary” parameters from the Cisco AP:

(Note: beacon interval is the time interval between two consecutive beacon frames)

Question 2

In general, what is the difference in transmit power between an AP operating at Tx Power Level 1 and the same AP operating at Tx Power Level 3?

A. 2 dB
B. 3 dB
C. 6 dB
D. 11 dB
E. 17 dB

 

Answer: C

Explanation

In Tx Power Level term, a level of “1” represents the highest power level available in the country for which this AP is configured. A level of “2” is 50 percent of this maximum, a level of “3” is 25 percent of this maximum, a level of “4” is 12.5 percent, and so on. Each level is half the power strength of the previous level.

Therefore, the difference between Tx Power Level 1 and Level 3 is calculated (in dB) as follows:

Difference (dB) = 10* log(25%) = -6 dB but we just need its amplitude-> C is correct.

Question 3

Which interface is considered a dynamic interface?

A. the virtual interface
B. the AP manager interface
C. the LAG interface
D. the management interface
E. the service port interface
F. a WLAN client data interface

 

Answer: F

Explanation

Each interface performs specific roles within the unified wireless environment. The static interfaces perform system roles, while the dynamic interfaces are user-defined, and provide client connectivity. Therefore a WLAN client data interface is considered a dynamic interface.

Question 4

An indoor 1240 AP is booting and has obtained an IP address using DHCP. The AP has confirmed prior controller IP addresses from a previously joined mobility group stored in NVRAM. What is the next step the AP performs?

A. DHCP option 43.
B. DNS resolution of CISCO-LWAPP-CONTROLLER.localdomain.
C. Layer 2 LWAPP discovery broadcast.
D. Layer 3 LWAPP discovery broadcast.
E. Ping each Cisco WLC to verify its current status.
F. Select a Cisco WLC based on its position in the configured join selection sequence.

 

Answer: A

Explanation

In a controller-based architecture, access points are dependent on a wireless controller to provide the software image, configuration, and centralized control and optionally data forwarding functions. Therefore, it is necessary for the access point to find a list of available controllers with which it can associate. Below is the process of finding as many controllers as possible in order to choose the best candidate to join after the AP has an IP address:

1 – LWAPP discovery broadcast on local subnet. This is a broadcast that is sent at Layer 2.
2 – OTAP
3 – Locally stored WLC IPs
4 – DHCP Option 43
5 – DNS Discovery.

DHCP protocol has become more and more popular over the time, hard and software vendors have started using the DHCP protocol to distribute all kinds of configuration options for their network devices or for their software. Now the problem is that the number of official options is limited and the process to get a new option recognized is difficult. For this reason, a new mechanism was introduced to make it easier for vendors to distribute their own proprietary information without clashing with other vendors and without having to register new options each time they decide the need them. This is achieved through the DHCP option 43 (Vendor Specific Information). Now the DHCP server has a private table of DHCP options which is kept separately for each vendor. Each vendor is identified by a vendor string. Now when a DHCP client asks for vendor specific options it makes a request with option 43 using the vendor string and a list of requested vendor specific options.

The information provided by DHCP option 43 is presumably interpreted by vendor-specific code on the clients and servers. The definition of this information is vendor specific. The vendor is indicated in the vendor class identifier option. Servers not equipped to interpret the vendor-specific information sent by a client MUST ignore it (although it may be reported). Clients which do not receive desired vendor-specific information SHOULD make an attempt to operate without it, although they may do so (and announce they are doing so) in a degraded mode.

Question 5

Which AP to Wireless LAN Controller discovery process requires a previous association of the AP with a Cisco WLC?

A. AP priming
B. defining a master controller
C. DHCP Option 6
D. DHCP Option 43
E. over-the-air provisioning

 

Answer: A

Explanation

AP priming happens after an AP is associated with at least one controller. The AP then gets a list of other controllers that it can associate with from the one it is already associated with. These other controllers are part of a mobility group. This information then gets stored in NVRAM and can be used if the AP reboots. To contact these controllers, the AP sends a broadcast to the primary controller and all the other controllers in the group.

Question 6

Which of the following methods is NOT used to access an AP in autonomous mode?

A. Browse
B. Telnet
C. Auxiliary
D. Console

 

Answer: C

Explanation

Well, there is no clear explanation why we can’t access an AP in auxiliary mode. You can access an autonomous AP in four ways:
* Console to it using a console cable and the console port.
* Telnet into it if it has an IP address.
* Browse to it with a web browser if it has an IP address.
* SSH into it if configured (preferred over Telnet).

(Reference: CCNA Wireless Official Exam Certification Guide)