Simple definition
Administrative distance helps a FortiGate choose between routes learned from different sources.
Technical definition
Each candidate route has a distance that represents the preference of its source. For comparable destinations and prefix lengths, a lower distance is generally preferred, after which route-type-specific attributes can break ties.
What is it used for?
Determine which route is installed when the same prefix can be learned in more than one way.
Practical example
A backup static route can use a higher distance than the primary route so that it becomes active only when the preferred path disappears.
Common issues
- Backup distance is too low and the wrong route becomes active
- Confusion between administrative distance, route priority, and dynamic-routing metrics
- A route exists in configuration but is not selected in the active routing table
How it works / role
FortiGate builds route candidates from connected interfaces, static routes, and dynamic protocols. For a given prefix it compares the applicable preference criteria, including administrative distance, and installs the best usable path. A change in reachability or route attributes can trigger a new selection.
Related terms
Key takeaway: Exact behavior can vary by version, product, and architecture; always validate on the environment actually deployed.