Information about Opportunistic Key Caching
Opportunistic Key Caching (OKC) is an enhancement of the WPA2 Pairwise Master Key ID (PMKID) caching method, which is why it is also named Proactive or Opportunistic PMKID Caching. Just like PMKID caching, OKC works with WPA2-EAP.
The OKC technique allows wireless clients and the WLAN infrastructure to cache only one PMK for client association with a WLAN, even when roaming between multiple APs because they all share the original PMK that is used for the WPA2 4-way handshake. This is required to generate new encryption keys every time a client reassociates with APs. For APs to share the original PMK from a client session, they must all be under a centralized device that caches and distributes the original PMK to all the APs.
Just as in PMKID caching, the initial association to an AP is a regular first-time authentication to the corresponding WLAN, where you must complete the entire 802.1X/EAP authentication for the authentication server, and the 4-way handshake for key generation, before sending data frames.
OKC is a fast roaming technique supported by Microsoft and some Android clients. Another fast roaming method is the use of 802.11r, which is supported by Apple and few Andorid clients. OKC is enabled by default on a WLAN. This configuration enables the control of OKC on a WLAN. Disabling OKC on a WLAN disables the OKC even for the OKC-supported clients.
A new configuration is introduced for each WLAN in the controller in Cisco IOS XE Amsterdam 17.2.1, to disable or enable fast and secure roaming with OKC at the corresponding AP.