Configuring Single-Area OSPFv2

OSPF Router ID

A router ID can also be assigned using a loopback interface.

The IPv4 address of the loopback interface should be configured using a 32-bit subnet mask (255.255.255.255). This effectively creates a host route. A 32-bit host route does not get advertised as a route to other OSPF routers.

The example in the figure displays how to configure a loopback interface with a host route on R1. R1 uses the host route as its router ID, assuming there is no router ID explicitly configured or previously learned.

Note: Some older versions of the IOS do not recognize the router-id command; therefore, the best way to set the router ID on those routers is by using a loopback interface.