The correct answer isfast BSS transition protocol, which is IEEE 802.11r Fast Transition. Cisco describes 802.11r BSS Fast Transition as the mechanism used to provide seamless roaming for wireless clients by reducing the time required to roam between access points. It achieves this by allowing keying material to be prepared or cached so that the client does not perform a full authentication exchange every time it moves to another AP within the same mobility domain.

This is specifically relevant to Layer 2 roaming because the client remains in the same Layer 2 network while reassociating to a different BSS. Fast Transition reduces roam latency, which is critical for voice, video, collaboration, barcode scanners, and other real-time applications. Cisco also classifies fast secure roaming as the method used to accelerate client roaming when Layer 2 security is configured on the WLAN. Increased beacon intervals do not provide handoff acceleration. Optimized roaming helps with sticky-client behavior but is not the secure handoff protocol. Deferred probe response is an RF/client steering behavior, not an 802.11 Layer 2 roaming key-transition method. Reference topics:802.11r Fast BSS Transition, Layer 2 roaming, fast secure roaming, WLAN mobility, and Catalyst 9800 roaming design.