WESM zl Management and Configuration Guide WT.01.28 and greater

1-78
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 zl 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
zl 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 1-77) apply to roaming between
RPs adopted by different Wireless Edge Services zl Modules. Without special
provisions, the 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.
However, Wireless Edge Services zl Modules supports these mechanisms to facilitate
and speed roaming between RPs adopted by different modules:
■ PMK caching—enables fast roaming back to a module in a WLAN that requires
WPA/WPA2 with 802.1X.
A station disassociates from one of the module’s RPs and moves to an RP on a
different module. As far as the first module knows, the station has left the WLAN.
However, the module stores the station’s PMK. If the station returns to an RP
on the first module, the key is ready, and the station can quickly connect.
■ Pre-authentication—enables fast roaming for the first time to a module in a
WLAN that requires WPA/WPA2 with 802.1X.