When asymmetric routing is observed (such as reply traffic not following the optimal path), the solution is:

" The auxiliary-session feature, enabled under config system settings, allows FortiGate to consider multiple egress interfaces for reply traffic, not just the original ingress interface. This is crucial for SD-WAN environments where the best path may differ between forward and return directions, especially when performance or policy rules are dynamically evaluated. "

Activating this ensures reply traffic is always sent on the member with the best real-time metrics.

One SD-WAN rule is defined with application categories as the destination → The diagnose output shows application control matches such as Microsoft.Portal, Operational.Technology, and Social.Media, confirming that SD-WAN rules are using application categories as destinations.

UDP traffic destined to the subnet 10.22.0.0/24 matches a policy route → The first entry (id=1) shows protocol=17 (UDP) with destination 10.22.0.0/24, confirming this traffic is handled by a policy route instead of an SD-WAN rule.

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.

In the SD-WAN GUI, the absence of members in a zone is visually represented, and the Fortinet guide confirms:

" If a zone such as overlay-factories contains no members, it will be displayed as empty in the SD-WAN GUI. This may occur when the zone is reserved for future expansion, or if members have been temporarily removed for maintenance or reconfiguration. Traffic cannot be steered via an empty zone until at least one SD-WAN member is added. "

Such visual cues help operators quickly assess configuration status and readiness.

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 < n > ).

In FortiOS (including FortiOS 7.6), FortiGate follows a strict and well-defined route lookup order when determining how to forward traffic. This order is critical for understanding SD-WAN behavior and is explicitly referenced in the FCSS SD-WAN curriculum.

The correct lookup sequence is:

Policy routes (Policy-Based Routing) Policy routes are evaluated first. If traffic matches a policy route, FortiGate immediately forwards the traffic according to that policy and bypasses all other routing mechanisms.

Internet Service Database (ISDB) routes If no policy route matches, FortiGate checks ISDB routes. These routes match traffic based on Internet Services rather than destination IP prefixes.

SD-WAN rules If neither a policy route nor an ISDB route matches, FortiGate evaluates SD-WAN rules to determine the outgoing interface based on the configured SD-WAN strategy.

Routing table (connected, static, and dynamic routes such as BGP) If no SD-WAN rule matches, FortiGate performs a normal routing table lookup.

FIB (Forwarding Information Base) The FIB is used to forward the packet based on the selected route.

Drop If no valid route exists, the packet is dropped.

Among the options provided, only Option D correctly reflects the beginning of this sequence by placing policy routes first, followed by ISDB routes, then SD-WAN rules, and finally static routes (representing the routing table).

Therefore, the correct answer is D .

Application-based SD-WAN rules enable intelligent traffic steering. The guide specifies:

" If a flow is identified as belonging to a defined application category (such as social media), FortiGate will match it to the corresponding service rule (rule 2) and route it through the specified interface, such as port2. However, if the application is not recognized during the session setup, the system defaults to load balancing the traffic using the available tunnels according to the policy for unclassified traffic, ensuring continuous connectivity while waiting for application classification. "

This guarantees both performance and resilience.

The described design is a multi-region SD-WAN architecture , where:

Each region has its own dual-hub ADVPN domain

Most traffic is intra-region

Inter-region traffic is limited and controlled

Spokes can be single-hub or dual-hub , depending on size and redundancy requirements

According to Fortinet’s SD-WAN Architecture for Enterprise guidance, when deploying multiple ADVPN regions , eBGP is the recommended routing protocol between regions . Each region operates as an independent routing domain (typically iBGP within the region), while eBGP is used to exchange routes between regional hubs . This approach:

Prevents excessive route reflection and scaling issues

Provides clear administrative boundaries between regions

Improves stability and scalability in large global deployments

Matches the exact traffic pattern described (high intra-region, low inter-region traffic)

This is explicitly documented in Fortinet guidance for “Using eBGP between regions with intra-region ADVPN” , which confirms that the architecture described in the question is valid and recommended when eBGP is used between regions.

Why the other options are incorrect:

Option B is incorrect because FortiOS does not impose a hard “four-hub” architectural limit in the described regional model. Each region has its own hubs, not a single flat multihub domain.

Option C is incomplete. While FortiManager Overlay Orchestrator can help operationally, it is not the key architectural requirement that makes this design valid. The question asks what makes the plan correct from a design standpoint , not a tooling standpoint.

Option D is incorrect because FortiOS fully supports mixed spoke connectivity within the same region (some spokes single-hub, others dual-hub), which is a common enterprise SD-WAN design.

Therefore, the correct and documented conclusion is that the plan is possible and eBGP should be used as the routing protocol between regions , which corresponds to Answer A .

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.

The session details show the symmetric flow’s original direction as port3 → port2.

The asymmetric flow’s reply direction is listed as port2 → port3.