I had always thought the BGP path selection algorithm whittles down the alternative paths by elimination until only one candidate remains. But according to Wendell Odom's CCIE Certification Guide, page 445, that is not so. It appears that each of the 12 steps (or however many there are) is considered seperately. If any step fails to produce a unique clear winner, then all the paths are carried forward to the next step. That is, unless there is a clear winner, the “non-best†paths are not eliminated.
This has some strange consequences that I am I having a hard time getting my head around. One example is in my blog:
http://dorreke.wordpress.com/2008/04/02/gobsmacked-bgp-route-selection-algorithm/
Can anyone with BGP experience comment?
Kevin Dorrell
Luxembourg
