Route Selection

A local BGP router uses the following primary criteria to select a route from the routing table for the forwarding table:

1. Next-hop accessible—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. If multiple routes have the same MED value, route selection continues. (For more information, see Multiple Exit Discriminator.)

If more than one route remains after all these criteria are evaluated, the local BGP router evaluates a set of secondary criteria to select the single route to a destination for its forwarding table. The secondary criteria include whether the route was learned through an EBGP or IBGP, the IGP route metric, and the router ID.