Assuming there are open connections through an active system's NAT and a failover occurs, by default, what happens to those connections.

Open NAT connections are not lost by default because BIG-IP mirrors NAT connection table entries to the standby, enabling seamless session continuation after failover.

F5 BIG-IP replicates NAT connection state to the standby device by default as part of its built-in connection mirroring for NATs, meaning the newly active standby already holds the session state for all open NAT connections. This allows the newly active device to continue handling those sessions immediately after failover without requiring clients or servers to re-establish connections.

NAT connection mirroring is active by default and does not require manually enabling a 'Mirror' option or performing a pre-failover synchronization step - that manual requirement applies to virtual server connection mirroring, not NATs.

BIG-IP does not distinguish between long-lived and short-lived NAT connections for failover continuity; the default mirroring behavior covers all open NAT sessions regardless of protocol or connection duration.

Existing NAT connections are not dropped and re-initiated by the new active device; the mirrored connection state allows the standby to seamlessly continue all existing NAT sessions the moment it becomes active.