Recently, I've been working on a Pi router build with multiple 2.5 Gbps Ethernet ports using Radxa's Dual 2.5G Router HAT.
I wanted a simple way to check on total network TCP throughput using both interfaces (or really, as many interfaces as possible) to multiple computers on my network, and I noticed iperf3's --bind option (like --bind [ip address of interface]) was not splitting the traffic on both interfaces—it would just route all traffic through one!
Luckily, I found the issue Failing to bind to interface when multiple interfaces are present, and in it, @bmah888 mentioned the --bind-dev option, which is new as of iperf 3.10+.
Using that option (like --bind-dev [interface name]), you can force an instance of iperf3 to bind to one particular device. For example, assuming I have two servers on my network running iperf3 -s, I can run the following on the Pi to saturate both connections as much as the Pi will allow:
iperf3 --no-delay -c 10.0.2.5 --bind-dev eth1 --interval 0 --parallel 1 --time 30 &
iperf3 --no-delay -c 10.0.2.6 --bind-dev eth2 --interval 0 --parallel 1 --time 30 &
I've encountered some weird issues with multiple interfaces on iperf before, too—wrangling multiple connections in Linux (or any OS) can be tricky, since the OS usually has routing optimizations in place that interfere with your intentions! (See my Network interface routing priority on a Raspberry Pi blog post about that.