Fortinet FCSS_NST_SE-7.6 - Fortinet NSE 6 - Network Security 7.6 Support Engineer

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.

The correct answer is D .

The exhibit shows:

session_count=591

clash=162

memory_tension_drop=0

TCP sessions:

166 in NONE state

1 in ESTABLISHED state

3 in SYN_SENT state

2 in TIME_WAIT state

The study guide explains the TCP protocol states and states explicitly:

“When a session is closed by both the sender and receiver, FortiGate keeps that session in the session table for a few seconds, to allow for any out-of-order packets that might arrive after the FIN/ACK packet. This is the state value 5.”

In diagnose sys session stat, the exhibit shows 2 in TIME_WAIT state . Since TIME_WAIT = state value 5 , those are the sessions being kept briefly for possible out-of-order packets. That makes D correct.

Why the other options are wrong:

A is wrong because session_count=591 is the total number of sessions, while the TCP sessions shown add up to only 172 (166 + 1 + 3 + 2). So not all sessions in the table are TCP sessions.

B is wrong because the study guide says the number of sessions deleted because of low free memory is shown by memory_tension_drop , and in the exhibit it is 0 , not 162.

C is wrong because the study guide defines ephemeral/open TCP sessions as those not fully established , but the exhibit does not say all 166 in NONE state are specifically “waiting to complete the three-way handshake.” The clearest directly supported statement from the displayed states is the 2 TIME_WAIT sessions retained for out-of-order packets.

So the verified answer is: D .

The correct answers are C and D .

The study guide’s get vpn ipsec tunnel details example shows:

replay: enabled

inbound and outbound sections with separate SPIs

NPU acceleration: encryption(outbound) decryption(inbound) and it labels these as “Phase 2 SAs for each direction” and “Hardware acceleration”

This directly proves D. Anti-replay is enabled , because the output explicitly says replay: enabled

For the NPU status, the study guide explains the exact npu_flag meanings:

npu_flag=00 = both IPsec SAs loaded to the kernel

npu_flag=01 = outbound IPsec SA copied to NPU

npu_flag=02 = inbound IPsec SA copied to NPU

npu_flag=03 = both outbound and inbound IPsec SAs copied to NPU

Because the exhibit shows hardware acceleration in both directions — encryption(outbound) and decryption(inbound) — the matching npu_flag is 03 , not 02. That makes C correct and A incorrect.

Why B is wrong:

The same study guide output labels the tunnel as having Phase 2 SAs for each direction , so different inbound and outbound SPIs are normal for the two SAs. Also, the FortiOS administration guide explains that auto-negotiate controls whether phase 2 SA negotiation is initiated automatically, not whether inbound and outbound SPIs are different: “By default the phase 2 security association (SA) is not negotiated until a peer attempts to send data… Auto-negotiate initiates the phase 2 SA negotiation automatically…”

So the verified answers are: C, D .

The exhibit shows:

memory conserve mode: on

memory used: 2706 MB 89% of total RAM

memory used threshold red: 2675 MB 88% of total RAM

memory used + freeable threshold extreme: 2887 MB 95% of total RAM

The study guide states that the default thresholds are:

Extreme = 95%

Red = 88%

Green = 82%

So this FortiGate is in conserve mode because memory usage is 89% , which is above the red threshold (88%) , but it has not yet reached the extreme threshold (95%) .

The study guide then explains exactly what happens during conserve mode:

“For traffic that requires proxy-based inspection (and if memory usage has not exceeded the extreme threshold):

config system global

set av-failopen [off | pass | one-shot]

pass (default): All new sessions pass without inspection”

It also says:

“The av-failopen setting also applies to flow-based antivirus inspection.”

And the same page adds:

“If memory usage exceeds the extreme threshold, all new sessions that require inspection (flow-based or proxy-based) are blocked.”

Therefore, with default settings and with memory usage below the extreme threshold , FortiGate is allowing new sessions that require inspection, but bypassing inspection . That matches C .

Why the other options are wrong:

A is wrong because the default behavior is not to block proxy-based inspected sessions; the default is pass , meaning they pass without inspection

B is wrong because if memory rises another 6% , it reaches 95% , which is the extreme threshold . At that point, the study guide says all new sessions that require inspection are blocked

D is wrong because FortiGate blocks all new inspected sessions only when memory usage exceeds the extreme threshold , and the exhibit shows it is currently at 89% , not 95%

The correct answer is A . This behavior is dictated by the configuration command set snat-route-change enable shown in Exhibit 1 under config system global.

Routing Change: By changing the priority of route ID 2 from 10 to 0, it becomes lower than route ID 1 (priority 5). In FortiOS, a lower priority value indicates a more preferred route. Consequently, the active route for the destination changes from port1 to port2 .

SNAT Implication: The existing session (shown in Exhibit 2) is using Source NAT (SNAT) with the IP address associated with port1 (10.200.1.1). If the traffic were simply switched to port2 , the source IP would be incorrect for that interface and the return traffic would likely fail or be dropped.

snat-route-change enable: This specific setting instructs the FortiGate on how to handle established SNAT sessions when a routing change occurs that alters the preferred outgoing interface. When enabled, if a route change forces an SNAT session to a new interface, FortiGate flushes (deletes) the session from the session table. This is necessary because a live TCP session cannot survive a change in its source IP address. The client must initiate a new session, which will then be created using the new correct route (port2) and the corresponding new SNAT IP.

If this setting were disabled, the session would likely remain " sticky " to the original interface (port1) until it closed, provided the route still existed. However, the explicit configuration forces the deletion.

For Option A:In Fortinet BGP (and standard BGP), when a prefix is displayed with an " i " (lowercase i) in the Path column, it represents an internal prefix that originated from the local router, typically configured via the BGP " network " command. In the exhibit, the prefix 10.20.30.0/24 is listed with a Path value of i, indicating it was injected into BGP by the local router using the network statement, not via redistribution from another routing protocol. The same logic applies to i as documented: " Origin code ' i ' means the route was injected via the network command. "

For Option D:The get router info bgp network output is a summary table displaying both local and received BGP routes. It lists all known routes to the BGP process, whether received from peers or originated locally. The exhibit shows all BGP prefixes known to the local router, matching the official admin guide’s description of this command’s output.

Explanation for B and C:

The phrase “legacy route advertisement” is not formalized in BGP documentation or Fortinet’s admin guide; the output uses standard BGP mechanics.

If a route was redistributed into BGP from another routing protocol, the Path field would display a " ? " (question mark) for incomplete (redistributed) origin. Here the /24 route has " i " so it is NOT a redistribution.

The correct answers are A and B .

The exhibit shows:

override: disable

both members are currently in-sync

only port7 appears under HBDEV stats , so it is the active heartbeat interface

the cluster is in HA A-P mode

Why A is correct:

With override disabled , after a failover the new primary keeps that role when the old primary comes back. The FortiOS administration guide states:

“When the primary FortiGate rejoins the cluster the secondary FortiGate continues to operate as the primary FortiGate.”

So if FGVM...649 reboots and FGVM...650 becomes primary, FGVM...650 will remain primary after FGVM...649 rejoins.

Why B is correct:

The study guide states:

“When FortiGate devices configured in an HA cluster lose communication with each other on the heartbeat interface, each FortiGate assumes the role of the primary device.”

The exhibit shows only port7 as the heartbeat device in HBDEV stats

So if port7 is disconnected and heartbeat communication is lost, the cluster can enter a split-brain condition, where both units believe they are primary. The FortiOS administration guide confirms the same behavior: loss of heartbeat communication causes each member to think it is the primary

Why the other options are wrong:

C is wrong because configuration synchronization status is specifically used to detect whether secondary members remain synchronized with the primary. If members are no longer synchronized, the status changes from in-sync to out-of-sync

D is wrong because the study guide explains that during a configuration change, checksums may differ briefly while changes are copied, but it does not describe this as the secondary initiating a “synchronization reset”

So the verified answers are: A, B .