FortiGate evaluates route sources in a fixed precedence: it checks Policy routes first, then ISDB routes (a special kind of policy route defined by Internet Service), then SD-WAN rules, and only if none match does it consult the regular routing table/FIB. Regular policy routes take precedence over SD-WAN policy routes, which is why SD-WAN rules come third. Within the FIB, longest-prefix-match applies, so directly Connected routes (administrative distance 0, most specific) are preferred, while a Default route (0.0.0.0/0) is the gateway of last resort. Connected routes therefore fill the fourth slot, and Default routes is the excluded distractor because it is the least-preferred, most general entry.
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