Notes-Section : In OSPF, Loopbacks are always advertised as /32 except...

#Topology#

#Note#
In OSPF, Loopbacks are advertised as /32 as described in RFC2328 Section 9.1.
It is seemed as a stub host, because over a loopback, there will be no transit-traffic.

You can change this behavior with the ip ospf network point-to-point command on the Loopback-Interface. Have a look ->

#Config#
#Rack1R1#

Rack1R1#sh run int lo0
Building configuration...

Current configuration : 95 bytes
!
interface Loopback0
 ip address 150.1.1.1 255.255.255.0
 ip ospf network point-to-point
end

Rack1R1#
Rack1R1#sh ip ospf interface lo0
Loopback0 is up, line protocol is up 
  Internet Address 150.1.1.1/24, Area 0 
  Process ID 1, Router ID 150.1.1.1, Network Type POINT_TO_POINT, Cost: 1
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 0
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 0, Adjacent neighbor count is 0 
  Suppress hello for 0 neighbor(s)
Rack1R1#
Rack1R1#sh ip route 150.1.1.1
Routing entry for 150.1.1.0/24
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via Loopback0
      Route metric is 0, traffic share count is 1

Rack1R1#
Rack1R1#sh ip route 150.1.3.3
Routing entry for 150.1.3.3/32
  Known via "ospf 1", distance 110, metric 66, type intra area
  Last update from 164.1.13.3 on Serial0/1, 02:15:50 ago
  Routing Descriptor Blocks:
  * 164.1.13.3, from 150.1.3.3, 02:15:50 ago, via Serial0/1
      Route metric is 66, traffic share count is 1

Rack1R1#

#Rack1R3#

Rack1R3#sh run int lo0
Building configuration...

Current configuration : 63 bytes
!
interface Loopback0
 ip address 150.1.3.3 255.255.255.0
end

Rack1R3#
Rack1R3#sh ip ospf interface lo0
Loopback0 is up, line protocol is up 
  Internet Address 150.1.3.3/24, Area 0 
  Process ID 1, Router ID 150.1.3.3, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host
Rack1R3#
Rack1R3#sh ip route 150.1.1.1
Routing entry for 150.1.1.0/24
  Known via "ospf 1", distance 110, metric 782, type intra area
  Last update from 164.1.13.1 on Serial1/2, 02:17:59 ago
  Routing Descriptor Blocks:
  * 164.1.13.1, from 150.1.1.1, 02:17:59 ago, via Serial1/2
      Route metric is 782, traffic share count is 1

Rack1R3#sh ip route 150.1.3.3
Routing entry for 150.1.3.0/24
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via Loopback0
      Route metric is 0, traffic share count is 1

Rack1R3#