FCSS_SDW_AR-7.4 FCSS - SD-WAN 7.4 Architect Question and Answers

With the Best Quality strategy, FortiGate first checks which links meet the SLA targets defined in the selected performance SLA (in this case, Custom-profile-1). Among those qualified links, FortiGate then uses the member configuration order to decide preference. If multiple links still qualify, it finally considers the member local cost to select the best path.

In an SD-WAN rule, you can steer application traffic by using Internet Service Database (ISDB) entries. Facebook and LinkedIn are predefined ISDB objects in FortiGate, so the correct way is to select them in the Internet service field under Destination. This ensures that all traffic to these applications is matched and routed through the chosen (less costly) link.

The log shows messages on HUB1-VPN1 where the device processes a SHORTCUT_QUERY and performs NAT hole punching (peer at 192.2.0.1:4500). This indicates that the device is acting as a hub, helping two spokes (192.2.0.1 and 10.0.3.101) establish a direct ADVPN shortcut tunnel between each other, instead of routing their traffic through the hub.

This is because the setting minimum-sla-meet-members = 2 requires at least two SD-WAN zone members (in this case, HUB2-VPN1, HUB2-VPN2, and HUB2-VPN3) to pass the defined SLA health check (HUB1_HC) before the FortiGate will establish ADVPN shortcuts. If fewer than two members meet the SLA, shortcuts will not be created.

FortiManager enforces a strict distinction:

"Firewall policies must reference SD-WAN zones, not individual SD-WAN members, when used in conjunction with SD-WAN templates. Attempting to install a policy that references a specific member (interface) will result in a deployment error, as member-level targeting is not supported in SD-WAN policy abstraction. This enforces centralized policy consistency and proper SD-WAN operation."

Ensuring policies target zones allows FortiGate to dynamically select the optimal member.

The implicit SD-WAN rule serves as the final catch-all. Per Fortinet:

"Sessions matching the implicit SD-WAN rule do not have an SD-WAN service id, as they are not associated with any specific user-defined SD-WAN rule. Additionally, this occurs only when traffic fails to match any entry in the policy route table. This default handling guarantees connectivity while minimizing the risk of blackholed traffic."

Administrators can observe this in diagnostic outputs for troubleshooting.

This configuration demonstrates a typical IPsec setup for SD-WAN overlays where the hub side requires a manually defined tunnel IP address, and the spoke can be flexibly configured, including interoperability with third-party IPsec devices. As described in the Fortinet SD-WAN Architect Guide: “For some overlays, the tunnel interface IP is configured statically on the hub side, which allows more control over overlay subnetting and facilitates the use of user-defined overlay IP addresses. This approach is also a requirement for compatibility with non-FortiGate endpoints, such as third-party IPsec devices that may not support dynamic address assignment via IKE or proprietary mechanisms.” This enables hybrid SD-WAN environments and advanced designs involving external partners or cloud services. Overlay IP flexibility is critical for route control and segmentation.

The SD-WAN monitor’s table view in FortiManager provides a donut visualization plus a detailed table that shows each SD-WAN member’s status and SLA pass/miss, giving the per-member health view you’re after.

To allow spokes in different hub-and-spoke groups to establish ADVPN shortcuts, the hubs must be configured to forward and send ADVPN shortcut offers. The key required settings on the hub are auto-discovery-forwarder (for VPNs to hubs) and auto-discovery-sender (for VPNs to spokes). This ensures the hub can facilitate and advertise ADVPN shortcut offers between spokes.

When the member priority of a port is increased (e.g., port2 to 20), FortiGate evaluates existing sessions and applies “dirty” flags where applicable. The SD-WAN session management mechanism is described in detail: “Upon a change in SD-WAN member priority, all existing sessions using that member are marked as dirty. For SNAT sessions, the gateway information is updated to ensure future packets are routed through the newly preferred member, in this case, port1. This automatic re-evaluation allows SD-WAN to dynamically respond to topology or priority changes, maintaining optimal routing.” This is fundamental to seamless failover and session persistence in Fortinet SD-WAN, ensuring active flows are redirected based on updated priorities or health status.

The SD-WAN overlay template defines a zone for each underlay interface and moves the interfaces into those zones. This statement perfectly describes the likely sequence of events. The template, when applied, re-organizes the interfaces and zones, causing the existing firewall policy that relies on the old zone configuration to fail. This is the most plausible root cause.

Enforce Device Configuration is enabled and the blueprint applies the provisioning CLI templates. The LAN-interface script sets port1 and port2 to DHCP and assigns a static IP to port5 (using the branch_id variable). Therefore, when FortiManager pushes the blueprint, it updates the configurations of port1, port2, and port5 - and their IP addresses may change accordingly.

When referencing a zone in a static route, FortiGate’s behavior is described as:

"Referencing a zone in a static route causes FortiGate to install a static route for each member interface of the zone. This enables ECMP (Equal-Cost Multi-Path) and load balancing where supported and ensures that traffic can be steered over any valid zone member according to SD-WAN rules or standard routing."

This mechanism is fundamental to Fortinet’s implementation of SD-WAN and simplifies large, multi-interface deployments.

For a large-scale SD-WAN deployment (such as 1000 spokes) where ADVPN shortcut routing is required and some remote sites connect via FortiSASE, the recommended overlay routing configuration is BGP running on loopback interfaces, combined with dynamic BGP for ADVPN shortcut routing. This design leverages the scalability and resilience of BGP, allowing dynamic discovery and route exchange necessary for shortcut tunnels between spokes in ADVPN environments. Using loopback interfaces for BGP peering is considered best practice because it decouples routing protocol stability from physical link status, ensuring that if a physical underlay interface fails, the BGP session remains up as long as there’s an alternate path. With dynamic BGP, each spoke can efficiently learn the routes to other spokes and dynamically establish shortcuts, which is critical at this scale. This method also integrates smoothly with FortiSASE for remote connectivity to the SD-WAN hub, providing flexibility and centralized management.

In the exhibits, port2 is already assigned to the SD-WAN zone named Test. An interface can only belong to a single SD-WAN zone, so before you can add both port1 and port2 into the new SD-WAN zone Underlay, you must first delete the SD-WAN Zone Test to free port2.

The router policy explicitly denies traffic with source 10.0.1.128/25 (which includes 10.0.1.130) and destination 128.66.0.0/24 (which includes 128.66.0.125). Even though SD-WAN service 4 shows members (port1 and port2) alive and available for this traffic, the router policy is evaluated first and blocks it. Therefore, FortiGate drops the traffic flow.