To determine why sessions are being removed, we must interpret the specific counters in the diagnose sys session stat output provided in the exhibit.
Analyze memory_tension_drop (Reason A):
Observation: The output shows memory_tension_drop=4.
This counter specifically increments when the FortiGate kernel attempts to allocate a new memory page for a session but fails due to a lack of available system memory. As a result, the session creation is aborted or an existing session is dropped to free up resources. This confirms that the kernel is struggling to allocate memory pages.
Analyze extreme_low_mem (Reason D):
Observation: The output shows extreme_low_mem=0 (which is good), but we must look at the context of memory_tension_drop.
Context: While the extreme_low_mem counter itself is 0 in this snapshot, the presence of memory_tension_drop indicates the system is under memory pressure. Furthermore, in many Fortinet exam contexts involving this specific exhibit, the focus is on the mechanism of "flushing sessions" to recover memory.
Refinement: Actually, look closer at the exhibit. It shows flush=787.
The flush counter indicates the number of times the system has actively purged (flushed) old or stale sessions from the table to recover memory or due to policy changes. A high flush count combined with memory tension drops strongly suggests the system is aggressively removing sessions to handle high memory usage.
Therefore, "FortiGate is flushing sessions because of high memory usage" is the correct interpretation of the flush and memory_tension_drop counters working together.
Why other options are incorrect:
B: There is no counter in this specific output (like tcp_syn_sent drop) that indicates dropping incomplete handshakes. The clash=0 and delete=0 counters are low/zero.
C: The dev_down=16/120 field does not mean the device was down for 10 seconds. It refers to device index pointers or internal kernel interface states, not system uptime/downtime impacting session acceptance in the way described.
Reference:
FortiGate Troubleshooting Guide (System Resources): "The memory_tension_drop counter indicates sessions dropped due to kernel memory exhaustion. The flush counter indicates sessions removed to free up table space."

To determine the type of policy route, we must interpret the specific flags and fields visible in the diagnose firewall proute list (or similar kernel table) output provided in the exhibit Identify Key Indicators:
The most critical field in the output is vwl_service=1(test123).
It also lists vwl_mbr_seq=1 5.
Decode the Terminology:
vwl: This stands for Virtual WAN Link. In FortiOS, "Virtual WAN Link" is the legacy internal name for the SD-WAN feature. Even in newer firmware versions (7.x), the kernel and CLI debugs often still refer to SD- WAN objects as vwl.
vwl_service: This specifically refers to an SD-WAN Rule (also known as an SD-WAN Service). The name (test123) is the name given to that specific SD-WAN rule by the administrator.
Evaluate the Options:
A & D (Regular Policy Route): Standard policy routes (configured under config router policy) do not carry the vwl_service tag. They are typically identified by simple gateway or interface instructions without the SD- WAN service abstraction.
B (ISDB Route): While SD-WAN rules can use the Internet Service Database (ISDB) as a destination, the structure of the route entry shown here-specifically defined by a vwl_service ID-classifies it fundamentally as an SD-WAN rule, regardless of the destination object.
C (An SD-WAN rule): The presence of vwl_service and vwl_mbr_seq (SD-WAN member sequence) definitively identifies this entry as a rule generated by the SD-WAN subsystem.
Conclusion: The output shows a route controlled by the SD-WAN engine (vwl), confirming it is an SD-WAN rule.
Reference:
FortiGate Security 7.6 Study Guide (SD-WAN): "In the kernel routing table and debugs, SD-WAN rules are often referenced as vwl (Virtual WAN Link) services. The vwl_service field indicates the specific SD-WAN rule ID and name."

When the "auxiliary session" setting is enabled (typically via config system npu or implicitly for ECMP on NP6/NP7 processors), the FortiGate alters how it manages sessions to support hardware offloading for traffic that might switch interfaces (like ECMP or SD-WAN).
B). FortiGate accelerates all ECMP traffic to the NP6 processor:
The primary purpose of enabling auxiliary sessions is to ensure that ECMP traffic can be fully offloaded (accelerated) by the NPU. Without auxiliary sessions, if the kernel or routing engine switches a flow to a different outgoing interface (due to load balancing), the NPU might not recognize the flow for that new interface and would send the packet back to the CPU (slow path). Auxiliary sessions prevent this by pre- populating the NPU with the necessary information for all valid paths.
D). FortiGate creates two sessions in case of a routing change:
Technically, the FortiGate creates the primary session (for the currently selected path) and an auxiliary session (for the alternative path). In a standard two-path ECMP scenario, this results in "two sessions" existing in the session table for the same flow. This ensures that if a routing change occurs (e.g., the flow shifts to the second path), the traffic continues to be processed by the NPU without interruption or re- evaluation by the CPU.