拓扑图：
AR1
inter g0/0/0
ip add 20.20.20.2 30
q
inter g0/0/1
ip add 10.10.10.1 30
q
inter g0/0/2
ip add 1.1.1.1 30
q
ospf 1 router-id 1.1.1.1
import-route static
import-route direct 静态直连，能学到20网段
area 0.0.0.0
network 1.1.1.0 0.0.0.3
network 10.10.10.0 0.0.0.3
ip route-static 192.168.3.0 255.255.255.0 20.20.20.1
AR2
inter g0/0/0
ip add 10.10.10.2 30
q
inter g0/0/1
ip add 2.2.2.2 30
q
inter g0/0/2
ip add 192.168.1.1 24
q
ospf 1 router-id 2.2.2.2
import-route direct
area 0.0.0.0
network 2.2.2.0 0.0.0.3
network 10.10.10.0 0.0.0.3
AR3
inter g0/0/0
ip add 1.1.1.2 30
q
inter g0/0/1
ip add 2.2.2.1 30
q
inter g0/0/2
ip add 192.168.2.1 24
q
ospf 1 router-id 3.3.3.3
area 0.0.0.0
network 1.1.1.0 0.0.0.3
network 2.2.2.0 0.0.0.3
network 192.168.2.0 0.0.0.255
AR4
inter g0/0/1
ip add 20.20.20.1 30
q
inter g0/0/0
ip add 192.168.3.1 24
q
ip route-static 192.168.1.0 24 20.20.20.2
ip route-static 192.168.2.0 24 20.20.20.2
ip route-static 10.10.10.0 30 20.20.20.2
ip route-static 2.2.2.0 30 20.20.20.2
ip route-static 1.1.1.0 30 20.20.20.2
测试：
PC1 ping PC3
PC2 ping PC1
PC2 ping PC3
最后PC2ping20.20.20.1也能实现