Border Gateway Protocol (BGP) uses a complex method of selecting the best path to the destination. The following list displays the criteria used by BGP for path selection:
1. Highest weight
2.Highest local preference
3.Locally originated paths over externally originated paths
4. Shortest autonomous system (AS) path
5. Lowest origin type
6. Lowest multiexit discriminator (MED)
7. External BGP (eBGP) paths over internal BGP (iBGP) paths
8. Lowest Interior Gateway Protocol (IGP) cost
9. Oldest eBGP path
10. Lowest BGP router ID (RID)

When determining the best path, a BGP router first chooses the route with the highest weight. Weight is a Ciscoproprietary BGP path attribute that is significant only to the local router; it is not advertised to neighbor routers. To configure the weight value, you should issue the neighbor {ipaddress | peergroupname} weight weightvalue command, where ipaddress is the IP address of a neighbor router, peergroupname is the name of a BGP peer group, and weightvalue is a locally significant weight value from 0 through 65535. By default, routes generated by the local router are assigned a weight of 32768 and routes learned from another BGP router are assigned a weight of 0.

When weight values are equal, a BGP router chooses the route with the highest local preference. The local preference value is advertised to iBGP neighbor routers to influence routing decisions made by those routers. To configure the local preference, you should issue the bgp default localpreference number command, where number is a value from 0 through 4294967295.
When local preferences are equal, a BGP router chooses locally originated paths over externally originated paths. Locally originated paths that have been created by issuing the network or redistribute command are preferred over locally originated paths that have been created by issuing the aggregate-address command.

If multiple paths to a destination still exist, a BGP router chooses the route with the shortest AS path attribute. The AS path attribute contains a list of the AS numbers (ASNs) that a route passes through.

If multiple paths have the same AS path length, a BGP router chooses the lowest origin type. An origin type of i, which is used for IGPs, is preferred over an origin type of e, which is used for Exterior Gateway Protocols (EGPs). These origin types are preferred over an origin type of , which is used for incomplete routes where the origin is unknown or the route was redistributed into BGP.

If origin types are equal, a BGP router chooses the route with the lowest MED. A MED value is basically the external metric of a route that is advertised to eBGP routers in order to specify a preferred path into an AS with multiple entry points. To configure the MED value, you should issue the defaultmetric number command, where number is a value from 1 through 4294967295. Routes redistributed into BGP are assigned this MED value; redistributed connected routes are assigned a
MED value of 0 regardless of the defaultmetric setting.

If MED values are equal, a BGP router chooses eBGP routes over iBGP routes. If there are multiple eBGP paths, or multiple iBGP paths if no eBGP paths are available, a BGP router chooses the route with the lowest IGP metric to the nexthop router. If IGP metrics are equal, a BGP router chooses the oldest eBGP path, which is typically the most stable path.

Finally, if route ages are equal, a BGP router chooses the path that comes from the router with the lowest RID. The RID can be manually configured by issuing the bgp router-id command. If the RID is not manually configured, the RID is the highest loopback IP address on the router. If no loopback address is configured, the RID is the highest IP address from among a router's available interfaces.