In the scenario described, where some APs did not failover to their secondary controller despite having sufficient licenses, the issue likely lies with the configuration of the APs to recognize the secondary controller.
Since the controllers are not in a mobility group but have A records for their hostnames in DNS, this suggests that they rely on DNS for AP discovery and failover. If APs are not failing over as expected, it could be due to an incorrect IP address configured for the secondary controller on the AP or within DNS. The exhibit shows that the "Secondary Controller" field has an IP address of "10.42.89.11", which should be verified against DNS records and actual controller settings to ensure accuracy. References := (CCNP Enterprise Wireless Design ENWLSD 300-425 and Implementation ENWLSI 300-430 Official Cert Guide Premium Edition and Practice)

The mesh backhaul link uses Advanced Encryption Standard (AES) for encryption. AES is a robust encryption standard that is widely used in various security protocols, including WPA2 and WPA3, to provide a high level of security for wireless communications. It ensures that the data transmitted over the mesh backhaul link is protected against unauthorized access and eavesdropping.
References: CCNP Enterprise Wireless Design ENWLSD 300-425 and Implementation ENWLSI 300-430 Official Cert Guide Premium Edition and Practice.

In a mobility group design, it is essential that all Wireless LAN Controllers (WLCs) share the same mobility domain name to ensure seamless client mobility and roaming between WLCs. This allows for clients to be recognized across the network and maintain their sessions as they roam. Additionally, when WLCs are in a Stateful Switch Over (SSO) high availability pair, they must use the same virtual IP address to present a single network entity to the wireless clients, ensuring uninterrupted service in the event of a failover.

The IEEE 802.11r protocol, also known as Fast BSS Transition (FT), is designed to support real-time applications by enabling faster roaming between APs. This protocol reduces the number of packets exchanged during the reauthentication process when a client moves from one AP to another, thus ensuring a more seamless transition and maintaining the quality of service required by real-time applications such as voice and video conferencing. References: Designing Cisco Enterprise Wireless Networks (ENWLSD 300-425)

Explanation:

https://www.cisco.com/c/en/us/td/docs/wireless/controller/7-4/configuration/guides/consolidated/b_cg74_CONS