Enterprise networks require enterprise-strength network monitoring solutions.
BMC Patrol, Hewlett-Packard OpenView and other such applications will monitor global networks and report to a central site whenever any server or router burps. Unfortunately, you need to be able to afford a global network to install, configure and maintain these packages. Installation intervals of six months or longer aren't uncommon, several man-months are needed to configure the software to check the network properly, and a continuing high level of work is required to maintain the network knowledge that must be imparted to the software. Still, the results are impressive when properly implemented. You can build your own out of bubble gum and shoelaces.