Beginning at 10:03 UTC, Cloudflare’ traffic monitoring systems detected a significant disruption impacting leading network providers worldwide.

For its part, CenturyLink said its IP NOC detected a bad Flowspec rule that propagated throughout the network.
The issue took approximately four hours to fully resolve.
https://blog.cloudflare.com/analysis-of-todays-centurylink-level-3-outage/