...

1. The first path received is automatically considered the 'best path'. Any further received paths are compared to the first received to determine if the new path is better.
2. Prefer the path with the highest WEIGHT.
   This parameter is not a part of the BGP standard, it is invented to quickly locally select the best route. A parameter is local to the router (assigned with routing filters in the BGP input) and cannot be advertised. A route without assigned WEIGHT has a default value of 0.
3. Prefer the path with the highest LOCAL_PREF.
   This attribute is used only within an AS. A path without the LOCAL_PREF attribute has a value of 100 by default.
4. Prefer the path with the shortest AS_PATH. (skipped if input.ignore-as-path-len set to yes).
   Each AS_SET counts as 1, regardless of the set size. The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the AS_PATH length.
5. Prefer the path that was locally originated via aggregate or BGP network
6. Prefer the path with the lowest ORIGIN type.

   Interior Gateway Protocol (IGP) is lower than Exterior Gateway Protocol (EGP), and EGP is lower than INCOMPLETE

   in other words IGP < EGP < INCOMPLETE
7. Prefer the path with the lowest multi-exit discriminator (MED).

   The router compares the MED attribute only for paths that have the same neighboring (leftmost) AS. Paths without explicit MED value are treated with MED of 0

8. Prefer eBGP over iBGP paths
9. Prefer the route that comes from the BGP router with the lowest router ID. If a route carries the ORIGINATOR_ID attribute, then the ORIGINATOR_ID is used instead of the router ID.
10. Prefer the route with the shortest route reflection cluster list. Routes without a cluster list are considered to have a cluster list of length 0.
11. Prefer the path that comes from the lowest neighbor address

Routing Filter Notes

On BGP output routing filters are executed before BGP itself is modifying attributes, for example, if nexthop-choice is set to force-self, then gateway set in the routing filters will be overridden.

On BGP input routing filters are applied to the received attributes, which means that, for example, setting gateway will work no matter what nexhop-choice value is set.

Running More than One Instance

...