Troubleshooting OSPF

We will be exploring some OSPF troubleshooting scenarios and see how to solve the problems.

Trouble ticket #1: Passive interface

We have two routers R2 and R3 configured as OSPF routers. The problem is that router R3 can not get routes that are redistributed into OSPF, from R2.
Let’s gather some information with the following debug commands:
debug ip ospf hello
debug ip ospf event

The list of debug messages appears on the console screen. As we can see, R3 is not receiving Hello packets from R2:
Let’s read the OSPF neighbor table with:

show ip ospf neighbor

Let’s check the OSPF interfaces on both routers with :

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

Now we see our routes in the routing table:

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