Before any LSAs can be sent a router must make itself unique and give itself a name so it can be differentiated apart from other OSPF neighbors in the area.
OSPF will choose or be assigned a 32 bit identifier referred to as the Router ID. (RID)
Here is the process a router uses to determine its RID without being specifically configured under the OSPF process:
- Use the router ID configuerd in the router-id subcommand under router ospf
- Use the highest numeric IP address on any currently nonshut loopback interface that has not been allocated under any other OSPF process
- Use the highest numeric IP address on any currently nonshut nonloopback interface thats not been allocated by another OSPF process.
Multiple OSPF processes running on a single router will choose unique RIDs for each OSPF instance.
The interface the RID is taken from does not have to be matched using an OSPF network command.
An interface can be in a down/down state to be considered to have a prospective RID
OSPF does not have to advertise the RID
The RID does not have to be reachable in the routing table.
RID is chosen when the OSPF process is started.
Routers consider the RID everytime the OSPF process is started or when its changed through configuration