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

The IKE_SA_INIT exchange in IKEv2 is responsible for DoS protection measures. During IKE_SA_INIT, before authentication and further exchange, the responder can use cookie challenges (per RFC 7296 and Fortinet VPN documentation). If a DoS attack is suspected (many requests from the same source), the responder replies with a cookie. Only after the initiator returns the correct cookie does the exchange proceed, protecting the responder from state exhaustion and certain forms of DoS traffic at the handshake stage.

To establish a functional Security Fabric, specific network and configuration prerequisites must be met to ensure nodes can communicate, authorize, and share telemetry data:

A. You must ensure that TCP port 8013 is not blocked along the way:

TCP port 8013 is the dedicated port for FortiTelemetry (Fabric) communication. If firewalls (intermediate or local) block this port, the Fabric connection between the root and downstream FortiGates will fail.

D. You must authorize the downstream FortiGate on the root FortiGate:

Security Fabric relies on a trust relationship. When a downstream device attempts to join, it appears in the Root FortiGate's dashboard. The administrator must manually authorize this device (unless pre-authorized via serial number) to allow it to join the Fabric topology.

E. You must enable FortiTelemetry on the receiving interface of the upstream FortiGate:

The interface on the Root (upstream) FortiGate that faces the downstream devices must have the "Security Fabric Connection" (formerly CAPWAP/FortiTelemetry) administrative access setting enabled. Without this, the interface will not listen for or accept Fabric connection requests.

Why other options are incorrect:

B: Neighbor Discovery uses standard multicast/broadcast or static settings; changing the port is not a standard requirement.

C: FortiGates can participate in the Security Fabric in either NAT or Transparent mode; Transparent mode is not a mandatory requirement for the Fabric itself.

The administrator is running a packet sniffer with the filter 'udp port 500 or udp port 4500 or esp'. The result is "no output," even though the VPN is attempting to establish (failing).

A. The VPN is configured to use IKE over TCP:

Standard IPsec IKE negotiation uses UDP port 500 (IKE) and UDP port 4500 (NAT-T).

However, if IKEv2 over TCP (RFC 8229) or Fortinet's proprietary IKE over TCP is configured (often used to bypass firewalls that block UDP), the traffic will use TCP (often port 4500 or 443).

The sniffer filter explicitly looks for udp or esp (IP Protocol 50).

If the traffic is encapsulated in TCP, it matches tcp protocol, not udp or esp (raw ESP). Therefore, the sniffer sees zero packets matching the filter.

Why other options are incorrect:

B: esp is a valid argument for diagnose sniffer packet. It is equivalent to filtering for IP protocol 50.

C: If the ISP were blocking traffic, the sniffer (running on the local FortiGate) would still see the outbound packets generated by the FortiGate trying to initiate the connection. "No output" implies the local device isn't even generating packets matching that filter.

D: Mismatched IKE versions would still generate IKE negotiation packets (proposals/errors) that would be captured by the sniffer.

Intermittent behavior (working sometimes, failing others) points to resource or connectivity fluctuations rather than static misconfigurations.

B. Check that FortiGate is not entering conserve mode:

Reason: When FortiGate enters Conserve Mode (due to high memory usage), it changes its inspection behavior to save resources. Depending on the av-failopen setting, it may either bypass inspection (allowing blocked sites) or drop traffic (blocking valid sites) temporarily until memory recovers. This flapping between states causes intermittent filtering issues.

D. Check that the communication between FortiGate and FortiGuard is stable:

Reason: The Web Filter engine relies on real-time queries to the FortiGuard Distribution Network (FDN) to categorize URLs that are not in the local cache. If the internet connection or the specific path to FortiGuard is unstable (packet loss, latency), queries will time out. This results in "Rating Errors," which can block or allow traffic unpredictably based on the "Allow websites when a rating error occurs" setting.

Why other options are incorrect:

A: A mismatch in inspection mode (e.g., Profile set to Proxy, Policy set to Flow) is a static configuration error. It would typically result in the profile not being selectable or consistently failing/not applying, rather than working intermittently.

C: If the wrong port is mapped (e.g., HTTP on 8080 is not mapped), the inspection engine will consistently ignore traffic on that port. It would not be intermittent.