We will be exploring some OSPF troubleshooting scenarios and see how to solve the problems.
Trouble ticket #1: Passive interface
debug ip ospf hello debug ip ospf event
show ip ospf neighbor
show ip ospf interface
We see that serial0/0.203 subinterface is a passive interface. Thus it is not exhcnaging OSPF Hello packets, which is problematic because R3 is on the other end of se0/0.203.
The solution is then to make the subinterface actively particiapting in OSPF Hello exchange with:
router ospf 1
no passive-interface ser0/0.203