Looking for a bandwith measuring tool.

For a project at work I'm looking for a (hopefully free) bandwith measuring tool that can tell me how much traffic flows between several subnets on a network. Netflow is not an option since our switches do not support it.

Reason: We're currently using a sase product for both SD-WAN and internet firewall, and I want to figure out how much bandwith is used by each. Offcourse our sase provider won't give that since they're paid by the megabit.