What is BGP? – BGP routing explained
There is already a nice writeup on the current incident from Cloudflare at https://blog.cloudflare.com/october-2021-facebook-outage/They key observations:"Due to Facebook stopping announcing their DNS prefix routes through BGP, our and everyone else's DNS resolvers had no way to connect to their nameservers. Consequently, 1.1.1.1, 8.8.8.8, and other major public DNS resolvers started issuing (and caching) SERVFAIL responses.But that's not all. Now human behavior and application logic kicks in and causes another exponential effect. A tsunami of additional DNS traffic follows.This happened in part because apps won't accept an error for an answer and start retrying, sometimes aggressively, and in part because end-users also won't take an error for an answer and start reloading the pages, or killing and relaunching their apps, sometimes also aggressively."