I would disagree; nagios is not limited to small systems... We're currently monitoring about 8500 services on 2834 routers with nagios quite successfully and have been doing so for nearly a decade now -- we started with Netsaint. With custom scripts receiving data from our inventory management system, Nagios config generation for 99% of the hosts is completely automated with only a handful of special cases that are hand-modified as needed. Our investment, both in initial/ongoing man-hours, hardware, etc is minimal so our ROI is decent too ;)
-- Marc
+1 / what he said I auto generate my nagios configs from an in house asset management system as well. It works great. Monitoring over 1k devices. We built a custom reporting system around nagios as well.