Information about Mobility Domain ID - 802.11i Roaming
A mobility domain is a cluster of APs forming a continuous radio frequency space, where the Pairwise Master Key (PMK) can be synchronized, and fast roaming can be enabled for 802.11r (Fast Transition) or 802.11i (WPA).
In the releases prior to Cisco IOS XE 17.2.1, the PMK cache was shared across the FlexConnect APs using the AP site tag. All the APs that are a part of a site tag share the PMK cache. This is applicable only for central authetication.
From Cisco IOS XE 17.2.1, you can create a Mobility Domain ID (MDID) for each of the APs. All the APs with the same MDID share the PMK cache keys even if they are in different site tags. When MDID is configured for APs, the PMK cache keys are not shared with the APs that are not a part of the same MDID, even if they are a part of the same site tag. MDID supports PMK cache distribution for both central authentication and local authentication.
The mobility domain can either be defined as a static configuration of clustered APs, all under a commonly configured MDID, or dynamically computed. You can implement a spatial clustering algorithm based on neighbor associations of APs. Each AP can only be a part of one roaming domain.
An MDID is used by 802.11r to define a network in which an 802.11r fast roam is supported. PMKs should be shared within mobility domains, allowing clients to support fast roaming. If defined, MDID takes precedence over a site tag.
MDID configurations are exercised only from open configuration models. For more information about open configuration models, see the https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/prog/configuration/172/b_172_programmability_cg.html.