Cisco IOS Classic Firewall Stateful Failover High Availability Solution
Source: Cisco Systems
Stateful Failover for the Cisco IOS Firewall allows a router to continue processing and forwarding firewall session packets after a planned or unplanned outage occurs. A backup (secondary) router automatically takes over the tasks of the active (primary) router if the active router loses connectivity for any reason. This process is transparent and requires neither adjustment nor reconfiguration of any remote peer.