Fast Roaming Explained: 802.11k, 802.11v and 802.11r

Voice and video clients do not fail because coverage is missing; they fail because the roam between two access points took 300 ms instead of 50 ms. Three standards fix that, and they are complementary rather than alternatives. 802.11k tells the client where it can go, 802.11v lets the infrastructure suggest when to go, and 802.11r makes the move itself fast. This article explains what each does and how to enable and verify them.

802.11k - Radio Resource Measurement

Without 802.11k, a client roaming decision is a full off-channel scan: probe every channel, hear every AP, then pick. With k enabled, the current AP hands the client a neighbour report listing nearby APs, their channels and their signal quality. The client scans a short list instead of the whole band.

• What it improves: scan phase, typically the largest part of roam time.
• Prerequisite: APs must know their neighbours, which means good RF planning and, in clustered controllers, correct cluster membership.
• Symptom when missing: every roam triggers a visible probe delay even with excellent coverage.

802.11v - BSS Transition Management

802.11v moves the decision to the network. Using a BSS Transition Management request, the infrastructure can suggest a better AP - for example to steer a client off an overloaded radio or off a channel it is stuck on. Clients that support v respond to these suggestions; clients that do not simply ignore them, so steering is a best-effort improvement rather than a guarantee.

In Aruba terms this is the mechanism Client Match uses to generate transition requests toward the 802.11k client list. The practical benefit shows up in high-density areas: sticky clients that would otherwise cling to the AP they first associated with get nudged to a closer one.

802.11r - Fast BSS Transition

802.11r, also called Fast BSS Transition (FT), pre-authenticates the client with multiple target APs in the same mobility domain, so the roam only needs the four-way handshake to establish transient keys instead of a full 802.1X exchange with the RADIUS server. That is where the largest latency saving comes from in WPA2/WPA3-Enterprise networks.

show ap debug client-table
show ap debug client-stats <client-mac>
show ap debug dot11r-ft-stats

Configure it per SSID: the mobility domain identifier must be consistent across the cluster, and every AP the client may roam to has to support the same configuration. Enabling r on some APs and not others produces clients that roam fast in one building and slowly in the next.

Design and Validation

• Enable k and v broadly - they are backward compatible and cost little.
• Enable r on all APs of the same SSID, or not at all; mixed deployments are worse than none.
• Check the packet path beyond the RF: single-client roams are also affected by switch access-port behaviour and PoE renegotiation on AP power-up. Station-side capture instructions are in packet capture with tcpdump on AOS-CX, PoE budgeting that matters when APs renegotiate power in PoE standards explained, and neighbour discovery across the wire in LLDP transmit/receive configuration.
• Measure, do not assume: capture the roam time from the client's perspective. A target of under 100 ms for FT roams is realistic on a well-tuned network.

原文链接：https://arubanetworking.hpe.com/techdocs/Archived/Instant-AOS-8/Instant_83_WebHelp/Content/Instant_UG/WLAN_SSID_conf/Support%20for%20dot11r.htm