Dropped, flooded, broadcast, multicast and L2 packets
In many evaluation or certification tests, FortiGate firewall is often required to log any packets dropped by the firewall. In most of cases, these packets are of invalid headers so firewall just drops them silently. It is natural to forward all these packets to IPS first so FortiGate firewall is able to generate logs for invalid packets.
Flooded, broadcast and multicast traffics do not reach any of services in the forwarding path. They can be inspected by the interface policy as long as they match the addresses defined. Potentially, L2 packets can also be sent to IPS for inspection through interface-policy, but it is not enabled in FortiOS 4.0.