R0:
ip route 172.16.2.0 255.255.255.0 172.16.3.1
ip route 172.16.1.0 255.255.255.0 172.16.3.1
R1:
ip route 172.16.4.0 255.255.255.0 172.16.3.2
ip route 172.16.1.0 255.255.255.0 172.16.2.1
R3:
ip route 172.16.4.0 255.255.255.0 172.16.2.2
ip route 172.16.3.0 255.255.255.0 172.16.2.2