I'm surprised this hasn't been quoted up til now.
United’s operations are running normally today following yesterday’s network outage. The outage lasted approximately two hours, and as a result we experienced 580 delays and nine cancellations. (8 yesterday, 1 residual today). The outage was caused when a piece of communication equipment in one of our data centers failed and disabled communications with our airports and web site. We have fully redundant systems and we are working with the manufacturers to determine why the backup equipment did not work as it was supposed to.
Source:
http://thebat-sf.com/2012/08/29/over...etwork-outage/
Sounds like a router or firewall device failed and the backup didn't work either.
The only comment I have here is that I would probably design entry points to the systems such that -
united.com
airport ops
reservations
all hit different points of entry with networked devices that back one another up in such a way that if one of these platforms has to be down, the others are not impacted.