The most popular network equipment monitoring systems distributed under license GPL – Cacti, Nagios, Zabbix are considered. Similarities and differences between them are revealed. The architecture and main components of the monitoring system are described. The working full-functional state of each considered system is modeled. Critical disadvantages of the compared monitoring systems are given. It is noted that due to the identifi ed shortcomings and the need to use a large number of third-party plugins for Cacti and Nagios, as well as due to the complexity of scaling these systems, Zabbix system is recognized as the best choice for monitoring a large number of metrics.