Explanation
This is the most likely cause of the issue where the access points in a Microbranch group are online in Aruba Central, but no VPN tunnels are forming. A Microbranch group is a group that contains both APs and Gateways and allows them to form VPN tunnels for secure communication. The VPN tunnels use GRE (Generic Routing Encapsulation) as the encapsulation protocol and IPSec as the encryption protocol. If there is a firewall blocking GRE traffic between the AP and the gateway, the VPN tunnels cannot be established. The other options are incorrect because they either do not affect the VPN tunnel formation or do not apply to a Microbranch group. References:
https://www.arubanetworks.com/techdocs/ArubaOS_86_Web_Help/Content/arubaos-solutions/gateways/microb
https://www.arubanetworks.com/assets/tg/TB_ArubaGateway.pdf

Explanation
Splitting a larger OSPF area into a number of smaller areas has several advantages for network scalability and performance. Some of these advantages are:
* It increases stability by limiting the impact of topology changes within an area. When a link or router fails in an area, only routers within that area need to run the SPF algorithm and update their routing tables. Routers in other areas are not affected by the change and do not need to recalculate their routes.
* It reduces processing overhead by reducing the size and frequency of link-state advertisements (LSAs).
LSAs are packets that contain information about the network topology and are flooded within an area.
By dividing a network into smaller areas, each area has fewer LSAs to generate, store, and process,
* which saves CPU and memory resources on routers.
* It reduces bandwidth consumption by reducing the amount of routing information exchanged between areas. Routers that connect different areas, called area border routers (ABRs), summarize the routing information from one area into a single LSA and advertise it to another area. This reduces the number of LSAs that need to be transmitted across area boundaries and saves network bandwidth.
References: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13703-8.html

* Proves knowledge of the PMK
* Exchanges messages for generating PTK
* Distributes an encrypted GTK to the client
* Sets first initialization vector (IV)

Explanation
Multi-Domain Authentication is the feature that should be enabled to support the requirement that the switch must be able to authenticate a single computer connected through a VoIP phone. Multi-Domain Authentication is a feature that allows an Aruba CX switch to apply different authentication methods and policies to different devices connected to the same port. For example, a VoIP phone and a computer can be connected to the same port using a single cable, but they can be authenticated separately using different credentials and assigned to different VLANs. The other options are incorrect because they either do not support multiple devices on the same port or do not provide authentication. References:
https://www.arubanetworks.com/techdocs/AOS-CX/10.05/HTML/5200-7540/GUID-7D9E9F6E-5C2A-4F7E-BE
https://www.arubanetworks.com/assets/tg/TB_ArubaCX_Switching.pdf

Explanation
The difference in the noise floor perceived by a client using a bonded 40 MHz wide channel as compared to an unbonded 20 MHz wide channel is 3 dB. The noise floor is the level of background noise in a given frequency band. When two adjacent channels are bonded, the noise floor increases by 3 dB because the bandwidth is doubled and more noise is captured. The other options are incorrect because they do not reflect the correct relationship between bandwidth and noise floor. References:
https://www.arubanetworks.com/techdocs/ArubaOS_86_Web_Help/Content/arubaos-solutions/wlan-rf/rf-fundam
https://www.arubanetworks.com/techdocs/ArubaOS_86_Web_Help/Content/arubaos-solutions/wlan-rf/channel-b