Multiple fibre routes to mission critical buildings are prety much the norm for many years now, even if the building owner doesnt spec it the comms provider is likely to insist. Sometimes there are more than two routes but they do need to be celalry seperated internally and externally.
CBSITCB gave a very eleoquant and concise descritpiton of non hrdware fail modes and software is useallya more difficult fix than hardware. However in a really mission critical situation then a huge amount of effort has to go into software resilience-think Airbus- and for that reason it would seem close to nonsensicle that misentered flight plans could cause such chaos.
I have worked with telecoms network managemnt and switching systems that had very high levels of resilience national or regional level of the telecoms network including in todays world intenet hardware reallycannot be allowed to fail. . To be fair they do not have the functional complexity of a system like NATs has msotly just switch paths around and maanging digital data flows. We did have one susyem that used a comparator function where two processors controlling one system constantly compared operating states. Ifa failure mode occured this would determien which system had in someway been corrupted and switched it out leaving the unchanged processor in charge.. However we did have a short outage due to the comparator failing so nothings completley foolproof . Personally i think the Daily Mail and Express sabotaged it to stop people travelling to Europe or vice versa