For iperf2, the -B doesn't bind to an interface but only sets the source ip
address in the packet. The routing tables decide the output interface.
https://sourceforge.net/p/iperf2/discussion/general/thread/f856ae2c/
Here's an article:
If memory serves me right, Marcel wrote:
> I'm trying to do an iperf3 test using a Huawei ME909 wwan modem on a
> raspberry pi zero w.
>
> I use the command as iperf3 -c 12.12.12.12 -t 3 -p 5201 -B 12.34.56.78
>
> where 12.12.12.12 is the ip of the server machine and 12.34.56.78 is the
> ip of
