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

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.

The SD-WAN rule’s preferred member is determined by packet loss comparison. As the documentation details:

"If all three SD-WAN members exhibit the same packet loss percentage, the preferred member can change according to secondary criteria such as cost, latency, or manual priority. However, if one member—such as HUB1-VPN3—matches the packet loss of other members, it can become the new preferred member as determined by the load balancing algorithm or fallback rules."

This behavior ensures fair utilization and redundancy.

Within ADVPN topologies, shortcut requests and responses traverse spokes and hubs. Fortinet documentation states:

"When a spoke receives a shortcut query from another spoke, it may forward the response to its hub for validation or to facilitate dynamic shortcut tunnel setup. This mechanism allows direct spoke-to-spoke communication for optimized routing and performance, reducing latency and offloading the hub after initial control-plane mediation."

This is a core benefit of ADVPN’s dynamic shortcut feature.

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.

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.

Traffic steering decisions in SD-WAN are based on both SD-WAN member priorities and route metrics. The guide states:

"If HUB1-VPN3 has a higher member configuration priority (lower numerical value) or a route with a lower priority value (which actually represents higher precedence in routing), it will be chosen over HUB1-VPN1, even if an SD-WAN rule would otherwise match HUB1-VPN1. These dual factors—member configuration and route table priority—govern traffic egress decisions."

Administrators should carefully align route and SD-WAN member priorities for predictable path selection.

MSSPs typically keep SD-WAN hubs in their infrastructure for cost and management efficiency. However:

"If the customer requires Secure Internet Access (SIA) with centralized breakout—meaning all internet-bound traffic must pass through the customer’s premises for compliance or inspection—the hub must be installed locally. Similarly, if there is a large volume of traffic between the customer’s branches that would overwhelm a cloud-based hub or require low-latency, local hub placement is necessary."

This ensures policy enforcement and optimal performance for specific customer needs.

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.