BGP
also called Border Gateway Protocol
The protocol by which autonomous systems advertise which address ranges they can reach, determining how traffic finds any destination on the internet.
Networks advertise prefixes to their neighbours; each network chooses among the paths it hears. There is no central authority — reachability is emergent from the advertisements, which is both what makes the internet resilient and what makes it fragile.
Architecturally it matters for three reasons. Anycast works by advertising the same prefix from many locations, so failover is a route withdrawal taking effect in seconds without client cooperation. Route withdrawal is a failure mode: Meta's 2021 outage occurred because its DNS servers withdrew their routes on losing backbone connectivity, making the whole domain unreachable. And route leaks and hijacks — accidental or malicious advertisement of someone else's prefix — periodically misdirect traffic, which is what RPKI origin validation exists to mitigate.
For most teams the practical exposure is via cloud dedicated connections, which use BGP for on-premises routing.