The loopback address provides stability for the OSPF process.
The loopback address specifies router ID for Router.
Before you enable an OSPF process, there must be at least one active interface with an assigned IP address. OSPF uses the highest IP address assigned to an active interface as the OSPF Router ID.
If loopback interfaces have been configured, then OSPF will use the highest loopback address as the Router ID even if the highest loopback IP address is smaller than the IP address of any active physical interface.
Using a loopback interface on an OSPF router is recommended because a loopback interface is never down.
A loopback interface will produce a stable OSPF router ID.
The network in Figure 1 demonstrates that the OSPF Router ID (RID) is the highest IP address assigned to an active physical interface. If a loopback interface is used, then OSPF will use the loopback IP address as the OSPF RID.
Start by removing all IP addresses and loopback interfaces from Router B. Now, attempt to configure an OSPF process on Router B.
RouterB#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
RouterB(config)#router ospf 1
OSPF: Could not allocate router id
OSPF cannot be enabled on Router B because OSPF needs a RID and there are no IP addresses assigned on Router B. Configure the serial interfaces on Routers A and B and then configure an OSPF process on Router B.
Router A
interface Serial0/1
bandwidth 64
ip address 10.1.1.1 255.255.255.252
clockrate 64000
_______________________________________________________________________
Router B
interface Serial0
ip address 10.1.1.2 255.255.255.252
bandwidth 64
router ospf 1
The configuration of the OSPF process on Router B was successful. Examine the OSPF RID on Router B using the show ip ospf command.
RouterB#show ip ospf
Routing Process "ospf 1" with ID 10.1.1.2
Supports only single TOS(TOS0) routes
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Number of DCbitless external LSA 0
Number of DoNotAge external LSA 0
Number of areas in this router is 0. 0 normal 0 stub 0 nssa
The only active interface on Router B is Serial0, so OSPF will use the IP address assigned to Serial0 for the router ID. Add a loopback interface to Router B and then re-examine the OSPF RID on Router B.
Router B
interface Loopback0
ip address 2.2.2.2 255.255.255.255
RouterB#show ip ospf
Routing Process "ospf 1" with ID 10.1.1.2
Supports only single TOS(TOS0) routes
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Number of DCbitless external LSA 0
Number of DoNotAge external LSA 0
Number of areas in this router is 0. 0 normal 0 stub 0 nssa
The OSPF RID has not changed. This is a stability feature of OSPF. The router ID will not change unless the OSPF process is restarted or if the interface used for the RID goes down. Shut down the serial interface on Router B, re-enable the serial interface on Router B, and examine the effect on the OSPF RID.
Verification
RouterB#show ip ospf Routing Process "ospf 1" with ID 2.2.2.2 Supports only single TOS (TOS0) routes SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Number of DCbitless external LSA 0 Number of DoNotAge external LSA 0 Number of areas in this router is 0. 0 normal 0 stub 0 nssa
Troubleshooting
