One of the failover units must have an Unrestricted license (UR), while the other can have a Failover
(FO) or UR license. Restricted units cannot be used for failover and two units with FO licenses cannot
be used in a single failover pair. The PIX 515, PIX 515E, PIX 525, and PIX 535 can be used for failover
if you have the optional Unrestricted (UR) license.