I do the test on a single computer. I have two gigabit ports on my motherboard. I isolate a port by putting it on another network namespace to do the test.

Client Network

iperf client: 192.168.2.2/24 router: 192.168.2.1/24

Server Network

router: 192.168.3.2/24 iperf server: 192.168.3.1/24

iperf Client

ip a add 192.168.2.2/24 dev enp9s0
ip l set up enp9s0
ip route add 192.168.3.1 via 192.168.2.1
iperf3 -c 192.168.3.1 --bidir -t 20

iperf Server

ip netns add iperfserver
ip link set dev eno1 netns iperfserver
ip netns exec iperfserver ip a add 192.168.3.1/24 dev eno1
ip netns exec iperfserver ip l set up eno1
ip netns exec iperfserver ip route add 192.168.2.0/24 via 192.168.3.2
ip netns exec iperfserver iperf3 -s