--R1--------R2--------R3---
                                             areo 0     areo 1
                            
R1#show running-config
!
interface Serial1/1
 ip address 10.0.0.1 255.0.0.0
 serial restart-delay 0
!
router ospf 1
 log-adjacency-changes
 network 10.0.0.0 0.0.0.255 area 0

R2#show running-config
!
interface Serial1/0
 ip address 10.0.0.2 255.0.0.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 20.0.0.1 255.0.0.0
 serial restart-delay 0
!
router ospf 2
 log-adjacency-changes
 network 10.0.0.0 0.0.0.255 area 0
 network 20.0.0.0 0.0.0.255 area 1

R3#show running-config
!
interface Serial1/0
 ip address 20.0.0.2 255.0.0.0
 serial restart-delay 0
!
router ospf 3
 log-adjacency-changes
 network 20.0.0.0 0.0.0.255 area 1

 

R1#show ip rou
Gateway of last resort is not set

O IA 20.0.0.0/8 [110/128] via 10.0.0.2, 00:01:11, Serial1/1
C    10.0.0.0/8 is directly connected, Serial1/1

R2#show ip rou
Gateway of last resort is not set

C    20.0.0.0/8 is directly connected, Serial1/1
C    10.0.0.0/8 is directly connected, Serial1/0

R3#show ip rou
Gateway of last resort is not set