Technical Tip: Static route with administrative distance 255

Description

This article will explain how FortiOS handles static route with administrative distance 255.

Scope

FortiGate

Solution

From GUI and CLI FortiOS will allow users to configure static route with administrative distance 255. Lets demonstrate blackhole route with an example:

However, distance 255 means that the route is invalid. The route will be visible in routing-table database.

But if this blackhole route will be only available route towards the destination, it will not be inserted into forwarding-table and as a result, traffic will not be dropped (routed) as expected.

Because of this, administrative distance of any static route needs to be 254 or less. Otherwise, route will be considered as unreachable.