Take a look at InterMapper https://www.helpsystems.com/products/network-monitoring-software
Not free but there is a trial and a "lite" version
I prefer Intermapper for my network monitoring.
https://www.helpsystems.com/products/network-monitoring-software
I use it to monitor utilization on the switch ports with SNMP and NetFlow/SFlow for firewalls. It usually gives me a pretty good view of how traffic is moving through the network.
We have used InterMapper for over 15 years.
It is very configurable. Features include:
Intermapper for network diagrams and mapping. https://www.helpsystems.com/products/network-monitoring-software
You can map 10 devices for free, and they have a free 30-day trial too. It's definitely on the more affordable end of the spectrum. Plus also does network monitoring and alerts.