The Stateful High Availability Upgrade is offered as an optional licensed feature. Only the primary unit in the HA pair needs to be licensed. With Stateful High Availability, the primary unit actively communicates with the backup on a per connection & per VPN level. As the primary creates & updates connection cache entries or VPN tunnels, the backup unit is informed of the changes. The backup unit remains in a continuously synchronized state so that it can seamlessly assume the network responsibilities upon failure of the primary unit with no interruption to existing network connections.