Hello @NaderH ,
you have configured NAT overload on R2.
The serial interface between R2 and R1 is configured with ip nat inside.
The serial interface between R2 and R3 is configured with ip nat outside
on R1 you just need a default static route pointing to R2's direct address
ip route 0.0.0.0 0.0.0.0 192.168.12.2
The use of OSPF between R2 and R3 with ip nat outside on R2's side can create issues.
Check if ospf adjacency is actually created on R2 using:
show ip ospf neighbor
From R1 pinging to R3's IP address on serial interface should be possible even if OSPF is broken between R2 and R3
180.103.15.1
For R1 to be able to ping all other R3 interfaces the OSPF between R2 and R3 has to be working state FULL/-
Lastly, it should be R3 and not R2 to inject a default route in OSPF domain because the subnet
210.108.10.0/24
is directly attached to R3 and advertised in OSPF area 1.
Hope to help
Giuseppe
