--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