Iftop and vnstat traffic monitoring not working correctly

Dear all,

I’m wondering why vnstat and iftop are not showing the correct throughput values. To be more specific: The “missing” traffic is transferred only between the lan-ports on the omnia. Is it possible, that those tools can only monitor traffic that goes through the cpu? My impression is, that lan-port-traffic is handled within the switch chip and the cpu/operating system does not know about this traffic.

If so, how can I change this behavior?

Thanks in advance!