By default the Intrusion Protection System (IPS) on a FortiGate unit is set to perform which action?

Blocking all network attacks is an active enforcement action that requires explicit configuration within an IPS profile and application to a firewall policy; it is not the default behavior.

Blocking the most common network attacks also requires specific configuration of IPS signatures and actions, which is not the default state of the IPS upon initial setup.

By default, if an Intrusion Protection System (IPS) profile is not applied to a firewall policy, or if the applied profile's signatures are configured to 'pass' or 'monitor' by default, traffic is allowed to flow through the FortiGate unit without active blocking by the IPS.

While traffic might be allowed, automatically logging all traffic specifically by IPS is part of configuration, and the most fundamental default behavior when IPS is not actively blocking is simply to allow traffic.