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Failover Licenses (8.3(1) and Later)
In Version 8.3(1) and later, failover units do not require the same license on each unit. For earlier versions, see the licensing document for your version.
Failover License Requirements
•Failover units do not require the same license on each unit.
Older versions of adaptive security appliance software required that the licenses match on each unit. Starting with Version 8.3(1), you no longer need to install identical licenses. Typically, you buy a license only for the primary unit; for Active/Standby failover, the secondary unit inherits the primary license when it becomes active. If you have licenses on both units, they combine into a single running failover cluster license.
•For the ASA 5505 and 5510 adaptive security appliances, both units require the Security Plus license; the Base license does not support failover, so you cannot enable failover on a standby unit that only has the Base license.
Here is the link if you need more info:
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC