Route Preferences

For unicast routes, the JUNOS routing protocol process uses the information in its routing table, along with the properties set in the configuration file, to choose an active route for each destination. While the JUNOS software might know of many routes to a destination, the active route is the preferred route to that destination and is the one that is installed in the forwarding table and used when actually routing packets.

The routing protocol process generally determines the active route by selecting the route with the lowest preference value. The preference value is an arbitrary value in the range from 0 through 4,294,967,295 (2³² – 1) that the software uses to rank routes received from different protocols, interfaces, or remote systems.

The preference value is used to select routes to destinations in external autonomous systems (ASs) or routing domains; it has no effect on the selection of routes within an AS (that is, within an IGP). Routes within an AS are selected by the IGP and are based on that protocol’s metric or cost value.

This section includes the following topics:

• Alternate and Tiebreaker Preferences
• How the Active Route Is Determined
• Multiple Active Routes
• Default Route Preference Values

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