" The result is that the gateway address will not be updated, unless the existing gateway is no longer valid "

The study guide identifies preserve-session-route as the configuration responsible for this behavior.

Technical Explanation:

The exhibit shows two WAN interfaces, port1 and port2, with static default routes.

Originally, both interfaces participate in routing with equal priority. The administrator changes the priority of port2 to 20, making port1 the preferred interface for new route selections.

However, the existing sessions were established using port2, and its interface configuration includes:

config system interface

edit " port2 "

set preserve-session-route enable

next

end

Option A is correct: The preserve-session-route setting enables interface stickiness for sessions using port2.

Because the original gateway and route remain valid, FortiGate preserves the existing sessions ' routing information and continues forwarding them through port2.

Option B is correct: Increasing the priority value on port2 makes it less preferred than port1. Consequently, new sessions that use the relevant default route are directed through port1.

Why the other options are incorrect:

Option C: FortiGate does not automatically update existing SNAT session gateway information merely because a more preferred path becomes available. The guide identifies snat-route-change as a separate setting used to permit appropriate SNAT route updates.

Option D: A route change can cause affected sessions to be marked dirty and reevaluated, but it is not accurate to conclude that every session is necessarily marked dirty or that dirty status overrides interface stickiness.

Option E: SNAT session dirty-state handling does not depend exclusively on using an IP pool. Interface-based SNAT sessions can also be affected by routing changes.

Architectural Consideration:

This distinction is important for SD-WAN failback design. Preserving established sessions can help maintain connection continuity, but it can also prevent traffic from returning to a preferred WAN interface until the session ends or the preserved route becomes invalid.