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. Reference: https://www.arubanetworks.com/techdocs/ArubaOS_86_Web_Help/Content/arubaos-solutions/gateways/microbranch.htm https://www.arubanetworks.com/assets/tg/TB_ArubaGateway.pdf