Wireless/Redundant Edge Services xl Module Management and Configuration Guide WS.02.xx and greater

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Introduction
Layer 2 and Layer 3 Roaming Between RPs and Modules
In other words, the module functions much like a single, high-capability AP
with many remote radios (the RPs). Therefore, when a station disassociates
from one RP and reassociates with another RP adopted by the same module,
the module already has in place the association, the authentication, and the
encryption keys. The roam is fast and seamless.
The Wireless Edge Services xl Module also supports these 802.11i mechanisms
for speeding up roaming in a WLAN that requires WPA/WPA2 with 802.1X:
■ Pairwise Master Key (PMK) caching —enables fast roaming
between RPs.
WPA’s TKIP (and WPA2’s CCMP) derive encryption keys from a unique
PMK for each association. This key is necessary for the station and the
module to communicate. PMK caching allows the module to store a
station’s PMK after the station disassociates with an RP so that the key is
still in place if the station then associates with a new RP.
■ Opportunistic key caching—facilitates fast roaming to a new RP.
The station attempts to use the same PMK that it used for one RP with
another. As long as the same module has adopted both of the RPs, the
PMK works.
No matter which security options you have enabled on a WLAN, a station
can quickly roam (under 50 milliseconds) between two RPs adopted by the
same module.
Roaming Between RPs on Different Wireless Edge
Services xl Modules at Layer 2
The same factors that affect roaming between APs (outlined in “Layer 2 and
Layer 3 Roaming Between RPs and Modules” on page 1-80) apply to roaming
between RPs adopted by different Wireless Edge Services xl Modules.
Without special provisions, the two modules operate as separate entities,
each forcing the user to complete all necessary steps to connect. In some
cases, the roam still occurs quickly and seamlessly, but in other cases it is
slowed by authentication requirements.