NPM polls static properties, such as interface speed and duplex mode, from the firewall devices using SNMP.
NPM also collects traffic statistics, such as bytes in and bytes out, from the firewall interfaces using SNMP or NetFlow. NPM then calculates bandwidth consumption based on the total amount of bytes coming through the firewall over a given time interval. Bandwidth consumption is expressed as a percentage of the interface speed. For example, if a firewall interface has a speed of 100 Mbps and NPM polls 10 MB of bytes in and 10 MB of bytes out in one minute, the bandwidth consumption is 20% [(10 MB + 10 MB) / 100 Mbps *
100%]1. References: Monitor firewalls with NPM, Calculate bandwidth utilization on an interface