OSPF Challenge: Mixing Numbered and Unnumbered Interfaces
Assuming you have the following configurations on R1 and R2:
R1 configuration
hostname R1
!
interface Loopback 0
ip address 10.0.0.1 255.255.255.255
!
interface Serial 0
encapsulation ppp
ip unnumbered Loopback0
ip ospf 1 area 1
!
router ospf 1
R2 configuration
hostname R2
!
interface Serial 0
encapsulation ppp
ip address 10.1.2.3 255.255.255.248
ip ospf 1 area 1
!
router ospf 1
What IP address can you use on the loopback interface of R1 to establish adjacency between R1 and R2? Can you use more than one IP address?
You can find the solution here.