A confirmation that a bridge should be placed in the 'up' state in order to function properly.
This is (bridge) br0. Is it up or down? If it's down, the broadcasts for that port
will be dropped and accounted.
A note on scaling networks (packet processing) in Linux:
In order to have multiple CPUs processing packets in parallel with
the kernel datapath, you need multiple queues with their interrupts
spread among all relevant CPUs. And in order to the NIC to spread
the traffic to different queues, you need different streams (5-tuple).
That is called RSS.
For example, stream A would go to queue#1 which is mapped to CPU#1 and
stream B would go to queue#2 which is mapped to CPU#2 and so on.
That is how Linux kernel works in general (as standalone NIC, or using
Linux bridge or OVS). Here is a doc explaining it in more details:
Linux Kernel Docs for Scaling -- with RSS (Receive Side Scaling), among other concepts, explained.