Originally published at https://blog.pathvector.dev/protocol-in-code-bgp-13/ — 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 a real Python file you can open, read, and run; the source lives at github.com/pathvector-studio/protocol-in-code. If you're newer to this and want to build up hands-on first, start with the companion Protocol Lab series, which is the practical, run-it-yourself track.
The question
When a BGP speaker receives news from the outside world, that news arrives in a few distinct flavors. A peer announces a prefix. A peer withdraws a prefix. A peer's session drops entirely. It's tempting to model all three as "an update happened, go recompute" — one generic entry point with a flag or two.
So here's the question to hold onto while you read:
