Route Selection

The BGP route selection process compares BGP attributes to select a single best path or active route for each prefix in the routing table. The attributes are compared in a particular order. A local BGP router uses the following criteria, in the order presented, to select a route from the routing table for the forwarding table:

1. Next-hop accessibility—If the next hop is inaccessible, the local router does not consider the route. The router must verify that it has a route to the BGP next-hop address. If a local route to the next hop does not exist, the local route does not include the router in its forwarding table. If such a route exists, route selection continues.
2. Highest local preference—The local router selects the route with the highest local preference value. If multiple routes have the same preference, route selection continues. (For more information, see Local Preference.)
3. Shortest AS path—The local router selects the route with the fewest entries in the AS path. If multiple routes have the same AS path length, route selection continues. (For more information, see AS Path.)
4. Lowest origin—The local router selects the route with the lowest origin value. If multiple routes have the same origin value, route selection continues. (For more information, see Origin.)
5. Lowest MED value—The local router selects the route with the lowest multiple exit discriminator (MED) value, comparing the routes from the same AS only. If multiple routes have the same MED value, route selection continues. For more information, see Multiple Exit Discriminator.
6. Strictly external paths—The local router prefers strictly external (EBGP) paths over external paths learned through interior sessions (IBGP). If multiple routes have the same strictly external paths, route selection continues.
7. Lowest IGP route metric— The local router selects the path for which the next hop is resolved through the IGP route with the lowest metric. If multiple routes have the same IGP route metric, route selection continues.
8. Maximum IGP next hops—The local router selects the path for which the BGP next hop is resolved through the IGP route with the largest number of next hops. If multiple routes have the same number of next hops, route selection continues.
9. Shortest route reflection cluster list—The local router selects the path with the shortest route reflection cluster list. Routes without a cluster list are considered to have a cluster list of length 0. If multiple routes have the same route reflection cluster list, route selection continues.
10. Lowest router ID—The local router selects the route with the lowest IP address value for the BGP router ID. By default, the router IDs of routes received from different ASs are not compared. You can change this default behavior. For more information, see the JUNOS Routing Protocols Configuration Guide.
11. Lowest peer IP address—The local router selects the path that was learned from the neighbor with the lowest peer IP address.

You can change the default behavior of some attributes (such as MED and router ID) used in the route selection process. For more information, see the JUNOS Routing Protocols Configuration Guide.