Import policy rewrites the route before best-path ever sees it

Originally published at https://blog.pathvector.dev/protocol-in-code-bgp-07/ — part of the free Protocol Lab series. This post is part of Protocol in Code , a free series that reads network protocols as logic — inputs, state, and branches — rather than as configuration examples. Every module points at one real Python file and asks you to read it the way you'd read any other code: what comes in, what mutates, where does control leave early. The source lives at github.com/pathvector-studio/protocol-in-code . Note: If you're newer to this and want to run things before you read things, start with Protocol Lab — the hands-on companion series that builds the muscle memory this one assumes. The question How does local import policy change or reject a path before best-path selection runs? That's the whole module in one line, and it hides a claim worth being suspicious of. Best-path selection in BGP is the famous part — the ordered tiebreaker list everyone half-remembers: highest weight, highest local_pref , shortest AS path, and so on. It's easy to treat that comparison as the decision point, as if routes arrive from peers and get ranked. They don't arrive and get ranked. They arrive, get rewritten , and then get ranked. Import policy is a function that runs between the wire and the comparison, and it has two powers: it can change the values the comparison reads, and it can make the candidate not exist at all. Which means the interesting question isn't "who won best-path" but "what did best-path actually receive." Read the code The file is src/protocol_in_code/bgp/import_policy.py . It's short enough to hold in your head all at once, which is the point — the shape is the lesson. Start with the policy object: @dataclass ( frozen = True ) class ImportPolicy : local_pref_override : int | None = None weight : int = 0 reject_next_hops : tuple [ str , ...] = () reject_invalid : bool = False Four knobs, and notice they're not four of the same thing. Two of them ( local_pref_overri