Route advertisement and FIB programming Que1. Assuming `bgp` is running on a network device, will the route advertisement and FIB programming(basically populating entries in hardware) occur in `parallel`? Or does it occur sequentially, meaning, first the FIB is programmed and then the routes are advertised? Que2. Ideally if a network device advertise the routes before FIB programming, this will lead to traffic blackholing/traffic getting dropped. Is that correct understanding? Que3. How about `withdraw` routes ?

To a certain point, the answer is "that depends on the implementation". However, it's _very likely_ that the forwarding table is updated _as soon as possible_ , while BGP updates have a slightly lower priority and are processed _afterwards_. As you've described, it doesn't make sense to advertise a route that's not working yet.

Removing routes follows the same logic - an obsolete route is removed from the FIB before a BGP update can be sent out and processed by the other peers. In extreme, that could lead to a temporary routing loop.