Second, the officials said, a command in the procedure triggered software flaws on the circuit card being upgraded and the switch started to loop. In this condition it sent out to the other switches a surge of false signals that swamped the network and eventually brought it to a halt.
Third, software installed to monitor system health failed to recognize the signal storm as being false messages and so did not cut off the offending source. The reason, said the officials, is that administrative messages of the sort sent out in last week's failure often are sent out in great volumes, especially after a node has gone down and is being brought back online.
The combination of the three problems caused serious problems in the network and received national attention.