Why was Facebook down for five hours?

Facebook was down for five hours last week. What happened and what do DNS and BGP have to do with it?

Facebook's explanation: https://engineering.fb.com/202....1/10/05/networking-t

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0:00 DNS
7:13 Caching DNS
10:34 Hop-by-hop routing
14:07 Default-free routing
18:28 Peering
19:50 BGP
26:08 The outage

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