I'm not a large scale ISP but I do have to monitor quite a lot of stuff. You'd be amazed at how much you can monitor with Nagios/Icinga(1,2). They are written

I'm not a large scale ISP but I do have to monitor quite a lot of stuff. You'd be amazed at how much you can monitor with Nagios/Icinga(1,2). They are written in C but call a lot of external stuff written in whatever you fancy and that's the power, right there. Bodge upon bodge! There's no single technology in these beasties. The interface between the system and the plugins is very basic to say the least, so you can throw whatever at it as you require. I'm a sysadmin not a programmer and need to get jobs done. We currently use Icinga 1 with a dash of Netdisco and I intend to migrate to Icinga 2 with DIrector etc. That said, I have dallied with OpenNMS many times ever since the project began - it's too good to ignore. Zabbix also turns my head quite often.