It happens when a provider advertises their internal routes to the world. It boils down to how BGP works where networks (called Autonomous Systems) broadcast r

It happens when a provider advertises their internal routes to the world. It boils down to how BGP works where networks (called Autonomous Systems) broadcast routes to all their peers (equal peers, customers, upstream providers, all peers.) These routes essentially say "for the IP prefix xyz.0.0.0/8, the AS 1234 is origin and you can get there in X hops through me". Problems happen when an AS broadcasts to one or more of their peers routes they don't originate or provide transit for. Network A might get a route to CloudFlare from 2 providers, one which is short and one that is long. If it accidentally tells the provider with the long route that it is a gateway toward the short route -- and if that provider believes it! -- then the provider with the long route will switch some or even all their traffic to go through network A and they might tell their own peers and so on. Then suddenly half of cloudflare's traffic goes through Qatar or dies trying...