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BGP preference, enabled by the prefer-in-winner-set command with the bgp keyword, means that, instead of selecting the winner randomly from all routes in the winner set (the last step in the selection algorithm), a BGP-selected route will take precedence over all non-BGP routes.
When BGP preference is enabled, the CNA system will limit the winner set to include the best BGP-selected route in the winner set plus any link that has as good or better performance score at the time BGP status is evaluated. If the two remaining routes are a BGP-selected route and a non-BGP-selected route and both have identical performance scores, random selection will determine the winner.
If the winner has already been narrowed to a single winner due to other criteria, then BGP status will not enter the equation.
Remember also that the CNA system will not move away from a currently asserted route if that route is a member of the winner set, regardless of any other preference settings or performance scores. The CNA decision maker process only makes a decision when the currently implemented route does not have a sufficiently good performance score to qualify as a member of the winner set.
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