The 802.11k standard allows clients to request neighbor reports containing information about known neighbor access points that are candidates for a service set transition. The use of the 802.11k neighbor list can limit the need for active and passive scanning.
The assisted roaming feature is based on an intelligent and client optimized neighbor list.
Unlike the Cisco Client Extension (CCX) neighbor list, the 802.11k neighbor list is generated dynamically on-demand and is not maintained on the controller. The 802.11k neighbor list is based on the location of the clients without requiring the mobility services engine (MSE). Two clients on the same controller but different APs can have different neighbor lists delivered depending on their individual relationship with the surrounding APs.
By default, the neighbor list contains only neighbors in the same band with which the client is associated. However, a switch exists that allows 802.11k to return neighbors in both bands.
Clients send requests for neighbor lists only after associating with the APs that advertize the RRM (Radio Resource Management) capability information element (IE) in the beacon. The neighbor list includes information about BSSID, channel, and operation details of the neighboring radios.
Assembling and Optimizing the Neighbor List
When the controller
receives a request for an 802.11k neighbor list, the following occurs:
- The controller searches the RRM neighbor table for a list of neighbors on the same band as the AP with which the client is currently associated with.
- The controller checks the neighbors according to the RSSI (Received Signal Strength Indication) between the APs, the current location of the present AP, the floor information of the neighboring AP from Cisco Prime Infrastructure, and roaming history information on the controller to reduce the list of neighbors to six per band. The list is optimized for APs on the same floor.
Assisted Roaming for Non-802.11k Clients
It is also possible to optimize roaming for non-802.11k clients. You can generate a prediction neighbor list for each client without the client requiring to send an 802.11k neighbor list request. When this is enabled on a WLAN, after each successful client association/reassociation, the same neighbor list optimization is applied on the non-802.11k client to generate the neighbor list and store the list in the mobile station software data structure. Clients at different locations have different lists because the client probes are seen with different RSSI values by different neighbors. Because clients usually probe before any association or reassociation, this list is constructed with the most updated probe data and predicts the next AP that the client is likely to roam to.
We discourage clients from roaming to those less desirable neighbors by denying association if the association request to an AP does not match the entries on the stored prediction neighbor list.
Similar to aggressive load balancing, there is a switch to turn on the assisted roaming feature both on a per-WLAN basis and globally. The following options are available:
- Denial count—Maximum number of times a client is refused association.
- Prediction threshold—Minimum number of entries required in the prediction list for the assisted roaming feature to be activated.
Because both load balancing and assisted roaming are designed to influence the AP that a client associates
with, it is not possible to enable both the options at the same time on a WLAN.