The current Smart Redundancy solution works with one Active and one Standby ZoneDirector.
The two ZDs can be located in the same Data Center, or physically separated if timing requirements
can be maintained. Here's some further details about SR from KBA-2269:
For Smart Redundancy, the Standby ZD will send heartbeats and the Active will send discovers,
at every 6s intervals. If after 15s, no reply is seen, the ZDs will make a decision if it is disconnected
with its peer, and the Standby ZD will change to Active.
When the two ZoneDirectors are communicating again, one Active ZD will change to Standby
role and an auto-synchronization process will be started. There is a timestamp to judge sync
from which ZD, to sync the latest one to the peer. The timestamp is updated by configuration
behavior. They will continue trying to communicate, sending discover msgs every 6s to peers,
until the ZDs are communicating again, when they will determine Active/Standby roles based
on 1) most managed APs, and/or 2) lower MAC address.