The issue where APs failed over to the tertiary controller instead of the secondary controller during a controller failure is likely due to the secondary controller being at capacity. When a controller reaches its maximum capacity, it cannot accept any additional APs, causing them to failover to the next available controller in the high availability configuration.

The Virtual Switching Link (VSL) is essential for communication between the active and standby Wireless LAN Controllers (WLCs) in a Stateful Switch Over (SSO) setup. If the VSL goes down, it can lead to a split-brain scenario where both WLCs assume they are active, causing network disruptions and inconsistencies in client handling. References: Designing Cisco Enterprise Wireless Networks (ENWLSD 300-425) Official Cert Guide.

IEEE 802.11ac Wave 2 APs are backward compatible with 802.11a/n clients, allowing these devices to connect and communicate with the newer APs, albeit at their respective maximum supported speeds.

During a client roaming event, the Wireless LAN Controller (WLC) is responsible for communicating the new Layer 2 Endpoint ID (EID) mapping of a wireless supplicant to the fabric domain. The WLC acts as a mobility orchestrator within the Cisco Digital Network Architecture (DNA) to ensure seamless roaming and connectivity. References: Designing Cisco Enterprise Wireless Networks (ENWLSD 300-425)
https://www.cisco.com/c/dam/en/us/td/docs/cloud-systems-management/network-automation-and-management/d

Optimized roaming is a feature that helps manage the client's roaming behavior by disassociating clients with a signal strength below a predefined threshold. This feature is designed to prevent clients from sticking to an AP with poor connectivity, which can lead to suboptimal performance. If the BSS Transition Management query is causing the client to roam and the disassociation imminent feature is not enabled, it indicates that optimized roaming is likely enabled, prompting the client to seek a better AP connection.