The switching module sent these malformed packets “as network management instructions to a line module,” and the packets “were delivered to all connected nodes,” the FCC said. Each node that received the packet then “retransmitted the packet to all its connected nodes.”

Source: How malformed packets caused CenturyLink’s 37-hour, nationwide outage | Ars Technica

But the outage continued because “the malformed packets continued to replicate and transit the network, generating more packets as they echoed from node to node,” the FCC wrote. Just after midnight, at least 20 hours after the problem began, CenturyLink engineers “began instructing nodes to no longer acknowledge the malformed packets.” They also “disabled the proprietary management channel, preventing it from further transmitting the malformed packets.”