OSPF IP Unnumbered

Using ip unnumbered on physical interfaces to borrow an IP address from a loopback interface is a valid configuration in OSPF. For example:

router ospf 1

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
!
interface Gi0/1
 ip unnumbered Loopback0
 ip ospf 1 area 0
!
interface Gi0/2
 ip unnumbered Loopback0
 ip ospf 1 area 0
 
interface Gi0/3
 ip unnumbered Loopback0
 ip ospf 1 area 0

However, on some platforms this doesn't work. For example, IOS 15:

R1(config)#interface gi0/1  
R1(config-if)# ip unnumbered Loopback0  
Point-to-point (non-multi-access) interfaces only

The advantage of ip unnumbered is that you don't need an IP address on every physical interface, thus you save IP addresses. The disadvantage is that you are unable to ping the physical interface because it doesn't have a unique IPv4 address.

OSPFv3 uses link-local addresses, which is a similar "solution" to ip unnumbered for IPv4.

Links

datatracker.ietf.org/doc/html/rfc5309