A ZoneDirector "cluster" is called Smart Redundancy, where you have two ZDs (that
can be on different switches/floors). One ZD is in Active mode, and one ZD is in
Standby mode. All APs are managed by the Active ZD, so Clients roaming between
APs will all be connected to your Active ZD, that maintains session information, so
that they will not have to re-authenticate.