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

Fortinet outlines key SD-WAN routing principles:

"Policy routes are always evaluated before SD-WAN rules, meaning if a policy route matches, SD-WAN steering is bypassed. If the best route for a destination is not via an SD-WAN member, SD-WAN rules do not apply, and members are ignored if they lack a valid route. This hierarchy ensures traffic always follows the most deterministic and valid path according to configuration."

Understanding these principles is critical for correct SD-WAN and routing integration.

The diagnose output lists SD-WAN members 4(HUB1-VPN1), 5(HUB1-VPN2), 7(HUB2-VPN1), 8(HUB2-VPN2), and 9(HUB2-VPN3). It does not include member 6 (HUB1-VPN3). From the template, HUB1-VPN3 is installed only on branch2_fgt and branch3_fgt - not on branch1_fgt. Therefore, the output must be from branch1_fgt.

Before FortiGate can steer traffic according to SD-WAN rules, certain configuration elements must be present. The guide states:

"SD-WAN is not a standalone feature and interacts with several fundamental FortiGate configurations. Specifically, you must: (1) Define the interfaces (physical, VLAN, or IPsec) that will act as SD-WAN members, (2) Create firewall policies to allow traffic to be steered by SD-WAN, and (3) Set up routing so that traffic has valid routes via SD-WAN members. Without these, SD-WAN rules will not be able to match or steer any traffic."

Security profiles and traffic shaping are not mandatory for basic SD-WAN steering but can be layered on for enhanced security and QoS once foundational elements are present.

When using the diagnose sys session list command, SD-WAN-specific session steering is indicated by the presence of the sdwan_service_id field in the session data. This identifier ties the session directly to a specific SD-WAN rule or service. As noted in the Fortinet documentation: “Sessions that are handled according to SD-WAN rules will include a service ID tag (sdwan_service_id) in their session listing. This allows administrators to correlate live sessions with SD-WAN policy matches for troubleshooting and visibility.” This is a crucial diagnostic tool, as it distinguishes between traffic managed by traditional routing and that explicitly controlled by SD-WAN steering logic, aiding in operational insight and troubleshooting.

The FortiManager SD-WAN overlay template preview, as described in the document, indicates:

"When the device is acting as a hub, the configuration enables the sending of ADVPN shortcut offers to spokes. This means the hub can facilitate on-demand dynamic shortcut tunnel creation between spokes, improving performance for branch-to-branch communication by bypassing the hub for inter-branch traffic after initial discovery."

Such a role is critical in scalable ADVPN topologies, enabling hub devices to optimize overlays dynamically.

Rules template → Defines the SD-WAN rules for traffic steering.

BGP template → Configures dynamic routing for overlay tunnels.

IPsec tunnel template → Builds the IPsec VPN tunnels from the spoke to the hubs.

The line sdwan_mbr_seq=1 sdwan_service_id=4 indicates that this session is part of an SD-WAN rule. sdwan_service_id=4 confirms that the session is being handled by SD-WAN rule ID 4. This directly links the flow to the SD-WAN configuration.

The line no_offload_reason: redir-to-ips denied-by-nturbo shows that the session is not offloaded to the NPU (Network Processing Unit) and is being processed by the main CPU. A session that is not offloaded can be re-evaluated. If the outgoing interface (the one currently being used) goes down, the FortiGate will re-evaluate the session against the SD-WAN rules to find a new active member to steer the traffic through. This is a fundamental behavior of SD-WAN, which ensures network resilience.

The hub must send additional paths to spokes (set additional-path send).

The hub must treat each spoke as a route-reflector client so spoke routes are reflected to other spokes.

The hub must specify how many additional paths to advertise (set adv-additional-path ).

The SD-WAN rule (config service edit 1) is configured with set mode priority. This means the rule selects the best interface based on a defined performance metric, as opposed to a simple static priority or SLA. The event log (image_41cfb5.png) shows Metric latency and Message Service prioritized by performance metric will be redirected in sequence order. This indicates that the rule is using latency to determine the preferred member. Given that the log message is about a change, and the most logical reason for a change in a priority mode is that a different member is now the best performer, it implies that the latency on port2 has become lower than that on port1.

The log message Service prioritized by performance metric will be redirected in sequence order confirms that FortiGate is changing the member being used for this service. Because the mode is priority, FortiGate dynamically selects the member that currently meets the best performance criteria, which in this case is latency. The log implies a new member has been selected as the most optimal, and with the default configuration, the members are sorted based on their performance, so the outgoing interface list is effectively updated to prefer the new best-performing member (port2).

In Fortinet SD-WAN, hubs should not have extensions like FortiSwitch, FortiAP, or FortiExtender installed, as these can affect hub functionality and scalability. While all device types can be included in the topology, the hubs must be "clean" FortiGate devices without such extensions to ensure proper ADVPN and overlay management.

When planning to deploy a large number of branches (e.g., 50), Fortinet recommends several preparatory steps to simplify and automate the rollout. Creating model devices allows you to predefine configurations and settings that can be cloned or adapted for each branch, saving time and minimizing manual errors. Preparing a Zero Touch Provisioning (ZTP) template enables automatic onboarding and provisioning of new FortiGates as soon as they come online, reducing manual intervention. Lastly, creating a policy blueprint allows for standardized policy deployment across all branches, ensuring consistent security and SD-WAN rule enforcement. This holistic approach streamlines the deployment process, allows for rapid scaling, and ensures that all devices are configured according to corporate policy from day one.

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.

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.

From the SD-WAN monitor in FortiManager:

"The SD-WAN monitor provides a summary view of the branch devices and their members. In the scenario shown, it is clear that branch2_fgt is missing SLA configuration for one member, as evidenced by the lack of performance metrics. The monitor also shows only two branches in the current topology, allowing quick assessment of branch health and configuration completeness."

This kind of visibility is vital for proactive monitoring and rapid troubleshooting in SD-WAN environments.

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.