The issue is caused by the strict matching logic of the configured Prefix List.
Current State: The rule is edit 1 with set prefix 172.16.0.0 255.255.0.0 and both ge (greater than or equal) and le (less than or equal) are unset.
Behavior: When ge and le are unset, FortiOS requires an exact match of the subnet mask. The current rule only matches the exact network 172.16.0.0/16. It denies 172.16.52.0/24 because the mask (/24) does not match the rule's mask (/16).
To fix this and inject 172.16.52.0/24, you must modify the list to match the /24 mask:
A). Add another entry to the prefix list to specifically allow the 172.16.52.0/24 network:
Creating a new rule (e.g., edit 2) with set prefix 172.16.52.0 255.255.255.0 will provide an exact match for the incoming route, allowing it to pass the distribute-list.
B). Change the ge value to 17:
By configuring set ge 17 on the existing rule (conceptually 172.16.0.0/16 ge 17), you change the logic from
"exact match" to "range match".
This configuration tells the router to match any prefix starting with 172.16.x.x that has a subnet mask length of 17 or greater.
Since the incoming route is a /24, and 24 is greater than 17, the route will match the prefix list and be accepted.
Why other options are incorrect:
C: The option text appears to read "Change the ... value to 16". If this refers to le 16, it would enforce the mask to be exactly /16 or less, which still excludes /24.
D: Changing the default behavior to implicit allow defeats the purpose of a filter (security control) and is not a standard configuration step for fixing a single missing route.
Reference:
FortiGate Security 7.6 Study Guide (Routing): "In prefix-lists, if ge and le are not used, the subnet mask must match exactly. To match subnets within a range, you must define the prefix length boundaries using ge or le."

To solve this high CPU usage scenario involving the ipsengine, we must understand the specific functions of the diagnose test application ipsmonitor commands shown in the troubleshooting steps.
Analyze the Situation:
Exhibit: The diagnose sys top output shows the ipsengine process is in a run state (R) consuming 99% CPU.
Previous Action: The administrator already ran diagnose test application ipsmonitor 5.
Result: The CPU usage did not drop.
Understand the Commands:
diagnose test application ipsmonitor 5: This command toggles IPS Bypass Mode. When enabled, the IPS engine lets traffic pass through without inspection.
Implication: If the CPU was high due to traffic volume, enabling bypass would drop the CPU load immediately.
Failure: Since the CPU remained at 99% after bypass, the ipsengine process is likely frozen, stuck, or in an internal infinite loop unrelated to the current traffic flow. The process itself is the problem, not the traffic volume.
Evaluate the Solution (Option B):
diagnose test application ipsmonitor 2: This command toggles the IPS engine's Enable/Disable status.
Because the engine is stuck (bypass failed to relieve pressure), the "Immediate action" required is to stop or restart the process entirely.
Running option 2 effectively disables/kills the stuck IPS engine instance, which will immediately drop the CPU usage to near zero. (It can then be toggled again to restart it).
Why other options are incorrect:
A (Reduce signatures): This is a tuning measure for normal operation, not an immediate fix for a stuck process at 99% CPU.
C (Disable IPS on policies): This is a configuration change that takes time and requires a commit; it is not the most immediate diagnostic tool available.
D (Bypass all IPS engines): This describes the action of command 5 (Bypass), which the prompt explicitly states was already performed and failed.
Reference:
FortiGate Security 7.6 Study Guide (IPS & Diagnostics): "Troubleshooting IPS high CPU: 1. Check top. 2.
Try bypass (ipsmonitor 5). 3. If CPU persists, restart the engine (ipsmonitor 99 or 2)."

When the FortiGate enters Conserve Mode due to high memory pressure (specifically reaching the Extreme Threshold at 95% memory usage, or the Red Threshold for proxy traffic), the system prioritizes stability and preventing a system crash (kernel panic).
D). FortiGate begins dropping all new sessions to protect resources:
In Extreme Conserve Mode (95%), the FortiGate kernel acts to preserve the remaining memory for system- critical tasks (like admin access and basic packet forwarding of existing sessions). To achieve this, it drops all new session initiation requests regardless of the inspection type.
In Red Conserve Mode (88%), it specifically drops new sessions that require proxy-based inspection (as these consume the most memory), while often still allowing flow-based traffic.
Among the provided choices, "dropping new sessions" is the only standard protective mechanism FortiOS employs to stop memory usage from climbing further.
Why other options are incorrect:
A: FortiGate does not automatically reboot in conserve mode; it attempts to recover by restricting traffic.
(Reboot is a last-resort crash, not a configured action).
B: Inspection modes (Proxy vs. Flow) are defined in firewall policies and cannot be dynamically switched by the system during runtime.
C: The system does not arbitrarily stop "non-essential processes" like logging or AV. Logging is critical for audit trails. While av-failopen can be configured to bypass scanning, the system typically defaults to "Fail- Close" (dropping traffic) rather than stopping the engines themselves.
Reference:
FortiGate Security 7.6 Study Guide (Diagnostics & Resource Usage): "When memory usage reaches the extreme threshold (95%), all new sessions are dropped to prevent memory exhaustion."