BGP Attributes - 1

How BGP finds the best path?

1. Attributes are ways that you can 'tag' incoming or outgoing BGP routes.
2. Some attributes are Well-known (everyone supports), while others are optional.
3. Some attributes are Mandatory (must be in the update), while others are Discretionary.
4. Some attirbutes are Transitive (travels from router to router), while others are Non-transitive.

• Well known attributes

           - Autonomous system path : How many AS do I have to go through to reach my destination?
                                                                                                                  (AS-path - Mandatory)
           - Next hop address (Mandatory)
           - Origin (Mandatory)

           - Local preference (discretionary)
           - Atomic aggregate (discretionary)

• Optional attributes

           - Aggregator
           - Multi-exit discriminator (MED/Metric)

     0. Ignore routes with an inaccessible next hop address.
     1. Prefer the path with the highest WEIGHT ( Cisco proprietary )
     2. Prefer the path with the highest LOCAL_PREF.    
▲Required that admin set something.
▼If admin didn't change the settings of above, the election of the routes begins from below.
     3. Prefer the path that was locally originated via a network command.
     4. Prefer the path with the shortest AS_PATH. (Usually, the route to the destination is decided here.)
     5. Prefer the path with the lowest origin type.
     6. Prefer the path with the lowest multi-exit discriminator (MED).
     7. Prefer eBGP over iBGP paths.
     8. Prefer the path with the lowest IGP metric to the BGP next hop.
     9. Determine if multiple paths require installation in the routing table for BGP Multipath.
     10. When both paths are external, prefer the path that was received first (the oldest one).
     11. Prefer the route that comes from the BGP router with the lowest router ID.
     12. If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.
     13. Prefer the path that comes from the lowest neighbor address.