To determine the cause of the failure, we must analyze the IKEv2 debug output provided in the exhibit (image_ad3dc6.jpg):
Identify the Negotiation Phase:
The debug log shows: responder received CREATE_CHILD exchange.
In IKEv2, the CREATE_CHILD_SA exchange is used to create new Child SAs (Phase 2) or to rekey existing ones.
The fact that the tunnel was previously " brought up successfully " implies the initial IKE SA (Phase 1) is stable, and this error is occurring specifically during a rekey event, which often involves Perfect Forward Secrecy (PFS).
Analyze the Proposals (The Mismatch):
Incoming Proposal (Remote Peer):
The remote peer sends a proposal containing two Diffie-Hellman groups: type=DH_GROUP, val=MODP2048 (Group 14) and type=DH_GROUP, val=MODP1536 (Group 5).
My Proposal (Local FortiGate):
The local FortiGate configuration expects: type=DH_GROUP, val=MODP3072 (Group 15).
Result of the Negotiation:
The debug output concludes with: no proposal chosen and Negotiate SA Error.
This error occurs because the local FortiGate cannot find a common Diffie-Hellman group between what it requires (Group 15) and what the peer is offering (Groups 14 or 5).
While this is technically a mismatch occurring during the Phase 2 (Child SA) creation, " A Diffie-Hellman mismatch " (Option A) is the precise root cause identified in the logs.
Why other options are incorrect:
B: The log shows received create-child request, confirming that UDP traffic is reaching the device and is not blocked.
C: The failure is in the CREATE_CHILD exchange (Phase 2/Rekey), not the IKE_SA_INIT or IKE_AUTH (Phase 1) exchanges.
D: While the mismatch is occurring within the Phase 2 definitions, Option A is the specific technical reason for the no proposal chosen error shown in the DH_GROUP lines.
Reference:
FortiGate Security 7.6 Study Guide (IPsec VPN): " Phase 2 parameters... if Perfect Forward Secrecy (PFS) is enabled, a Diffie-Hellman exchange is performed again. Both peers must match the DH Group. "

Based on the Fortinet FCSS - Network Security 7.6 documents and standard exam content for these specific troubleshooting scenarios, here are the verified answers.
Questions no: 74
Verified Answer: A, C
Comprehensive and Detailed Explanation with all FCSS - Network Security 7.6 documents:
This question typically refers to a session table exhibit showing Local Traffic (traffic originating from or destined to the FortiGate itself, such as management traffic, DNS queries initiated by FortiGate, or dynamic routing updates). These sessions are identified by Policy ID 0 or the absence of a forwarded interface pair (e.
g., local flag).
C). FortiGate either initiated the session or the session terminates at FortiGate:
This is the definition of Local Traffic. Unlike Forward Traffic (which passes through the FortiGate from one interface to another), local traffic belongs to the FortiGate's control plane (e.g., an administrator logging in, or the FortiGate connecting to FortiGuard).
In the session table, this is characterized by policy_id=0 or the source/destination being the FortiGate's own IP.
A). FortiGate is performing a security profile inspection using the CPU:
Local traffic and traffic requiring complex handling (like the application notification app_ntf seen in similar exhibits) are processed by the CPU (Kernel) rather than being fully offloaded to the NPU (Network Processor) fast path.
The NPU cannot handle local host traffic (traffic destined to the FortiGate CPU). Therefore, the CPU must process these packets.
Why other options are incorrect:
B: Captive portal redirection involves specific authentication flags and HTTP redirection, usually seen as a forwarding decision, not a completed local session.
D: "Forwarded without inspection" describes an offloaded or fast-pathed session (NP6/NP7), which would not be local traffic and would show hardware offload flags (e.g., np6_0).
Reference:
FortiGate Security 7.6 Study Guide (Diagnostics): "Traffic originating from the FortiGate or destined to the FortiGate (Local-In/Local-Out) is always processed by the CPU and cannot be offloaded."