administrative distance
termThe trust ranking a router applies when two routing sources offer the same prefix: lower wins, so connected beats static beats OSPF beats BGP-learned.
A local tiebreaker, not a protocol field: two vendors may rank the same sources differently.
RFC 1812 writes route selection as pruning rules applied in order: basic match, then longest match, then the metric, which is compared only between routes from the same routing domain, and only then vendor policy. Administrative distance is that vendor policy, so it never overrides prefix length: a /24 from a less trusted source still beats a /16 from a more trusted one, and a metric never settles a tie between two different protocols.
FRRouting prints the pair in brackets as distance and metric: [110/8] is an OSPF route of cost 8 and [115/20] an IS-IS route, and the detailed view spells it out as distance 110, metric 20. Its distance table starts with kernel and connected routes at 0 and static routes at 1.
Raising the distance turns a route into a fallback. FRRouting's static routes take an optional distance, and its documentation installs a route to Null0 at 255 so that, if the real next hops become unreachable, traffic for the prefix is dropped instead of following a less specific route such as the default.