BGP is how the ~75,000 networks that make up the public internet tell each other "I can reach prefix X via me." Each ASN runs BGP at its borders, exchanging route announcements with peers, customers, and upstreams.
BGP is policy-based, not path-cost-based: routers prefer announcements based on local preference, AS path length, and operator-configured rules — not raw latency. That's why your packets sometimes take a roundabout route between two nearby cities.
The trust model is loose: anyone with an ASN can announce any prefix. Famous outages (Pakistan-vs-YouTube 2008, Facebook 2021) trace back to BGP misconfigurations. Modern mitigations include RPKI for origin validation.
See also