Hello Fabrice
According to sources, RMON is a standard monitoring specification that enables various network monitors and console systems to exchange network-monitoring data. RMON provides network administrators with more freedom in selecting network-monitoring probes and consoles with features that meet their particular networking needs. RMON actually uses SNMP for both agent configuration and data collection, however it differs from SNMP in that it focuses more on “flow-based” rather than “device-based” management. RMON is actually similar to NetFlow and SFlow because the data collected deals mainly with traffic patterns rather than the status of individual devices.
Alternatively, PRTG leverages various protocols such as Ping, SNMP, WMI, NetFlow, jFlow and sFlow to name a few. Nagios also uses SNMP but in addition, can monitor POP3, HTTP, NNTP, ICMP, FTP, and SSH among others. There is nothing that restricts you using RMON in conjunction with any of these as they can function in parallel.
RMON can be configured with various queue sizes that hold packets for analysis by the RMON process. This can be increased, but depends on the available memory of the device in question. You can find out more information about this and other RMON configuration options in the following Cisco documentation:
I hope this has been helpful!
Laz