4.3 Configuring OSPF
4.3.2 Optional configuration commands
Configuring a Loopback Address
When the OSPF process starts, the Cisco IOS uses the highest local IP address as its OSPF router ID. If a loopback interface is configured, that address is used, regardless of its value. You can assign an IP to a loopback interface with the following commands:

router(config)#interface loopback number
router(config-if)#ip address ip-address subnet-mask

A loopback-derived router ID ensures stability because that interface is immune to link failure. The loopback interface must be configured before the OSPF process starts, to override the highest interface IP address.

It is recommended that you use the loopback address on all key routers in your OSPF-based network. To avoid routing problems, it is good practice to use a 32-bit subnet mask when configuring a loopback IP address, as shown:

router(config)#interface loopback0
router(config-if)#ip address 192.168.1.1 255.255.255.255

A 32-bit mask is sometimes called a host mask, because it specifies a single host and not a network or subnetwork. Note: To prevent propagation of bogus routes, OSPF always advertises loopback addresses as host routes, with a 32-bit mask.

Modifying OSPF Router Priority
You can manipulate DR/BDR elections by configuring the priority value to a number other than the default value, which is 1. A value of 0 guarantees that the router will not be elected as a DR or BDR. Each OSPF interface can announce a different priority. You can configure the priority value (a number from 0 to 255) with the
ip ospf priority command, which has the following syntax:

router(config-if)#ip ospf priority number

To set a router's E0 with a priority of 0 (so that it can not win DR/BDR elections on that network), you would use the commands shown here:

RTB(config)#interface e0
RTB(config-if)#ip ospf priority 0

For the priority value to figure into the election, it must be set before the election takes place. An interface's priority value and other key information can be displayed with the show ip ospf interface command as shown in the figure. The output in this example tells which routers have been elected the DR and BDR, the network type (in this case, broadcast multiaccess), the cost of the link (10), and the timer intervals specific to this interface. The timer intervals configured are Hello (10), Dead (40), Wait (40), Retransmit (5).