In the Junos OS architecture, the routing table often receives prefix information from various sources, including direct connections, static configurations, and multiple dynamic interior and exterior gateway protocols. Route preference, frequently referred to as administrative distance in other vendor environments, serves as the primary tie-breaking mechanism used by the Routing Engine to select a single " active " route when multiple entries for the exact same destination prefix exist from different protocol sources. Each routing source is assigned a default numerical value, where a lower numerical value indicates a more preferred or " trustworthy " source. For instance, a direct route typically carries a preference of 0, while OSPF internal routes default to 10 and BGP routes default to 170.

The selection process evaluates these values; the route with the lowest preference is installed in the forwarding table and used for transit traffic. If preferences are equal, Junos secondary tie-breakers like local preference or metric are considered. Understanding this hierarchy is critical for traffic engineering and ensuring predictable routing behavior across the fabric. Modification of these default values via routing policy allows administrators to influence path selection without altering the underlying protocol metrics themselves.