LibreNMS

https://www.librenms.org/

A fully featured network monitoring system that provides a wealth of features and device support.

Basic Info

Here's a brief list of supported features, some might be missing.

Auto discovery
Alerting
Multiple environment sensors support
Multiple protocols data collection (STP, OSPF, BGP etc)
VLAN, ARP and FDB table collection
Customizable Dashboards
Device Backup integration (Oxidized, RANCID)
Distributed Polling
Multiple Authentication Methods (MySQL, LDAP, Active Directory, HTTP)
NetFlow, sFlow, IPFIX (NfSen)
Service monitoring (Nagios Plugins)
Syslog (Integrated, Graylog)
Traffic Billing (Quota, 95th Percentile)
Two Factor Authentication
API
Auto Updating

Configuration

Additional Resources

https://docs.librenms.org/