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The original version of SonicOS Enhanced provided a basic High Availability feature where a Backup firewall assumes the interface IP addresses of the configured interfaces when the Primary unit fails. Upon failover, layer 2 broadcasts are issued (ARP) to inform the network that the IP addresses are now owned by the Backup unit. All pre-existing network connections must be rebuilt. For example, Telnet and FTP sessions must be re-established and VPN tunnels must be renegotiated.
Stateful High Availability (SHA) provides dramatically improved failover performance. The Primary and Backup appliances are continuously synchronized so that the Backup can seamlessly assume all network responsibilities if the Primary appliance fails, with no interruptions to existing network connections.