Explanation/Reference:
Explanation:
Here are the relevant parts of the router configs:

Continue checking their connected interfaces with the "show running-config" command:


The only difference we can see here is the line "ip ospf hello-interval 50″ on R3. This command sets the number of seconds R3 waits before sending the next hello packet out this interface. In this case after configuring this command, R3 will send hello packets to R5 every 50 seconds. But the default value of hello-interval is 10 seconds and R5 is using it. Therefore we can think of a hello interval mismatch problem here. You can verify with the "show ip ospf interface <interface>" command on each router.

So we can see both hello and dead interval are mismatched because the dead interval always four times the value of hello interval, unless you manually configure the dead interval (with the ip ospf dead-interval
<seconds> command).