Configuring OSPF loopback address and router priority

When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID. If there is no active interface, the OSPF process will not start. If the active interface goes down, the OSPF process has no router ID and therefore ceases to function until the interface comes up again.

To ensure OSPF stability there should be an active interface for the OSPF process at all times. A loopback interface, which is a logical interface, can be configured for this purpose. When a loopback interface is configured, OSPF uses this address as the router ID, regardless of the value. On a router that has more than one loopback interface, OSPF takes the highest loopback IP address as its router ID.

To create and assign an IP address to a loopback interface use the following commands:

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

It is considered good practice to use loopback interfaces for all routers running OSPF. This loopback interface should be configured with an address using a 32-bit subnet mask of 255.255.255.255. A 32-bit subnet mask is called a host mask because the subnet mask specifies a network of one host. When OSPF is requested to advertise a loopback network, OSPF always advertises the loopback as a host route with a 32-bit mask.

In broadcast multi-access networks there may be more than two routers. OSPF elects a designated router (DR) to be the focal point of all link-state updates and link-state advertisements. Because the DR role is critical, a backup designated router (BDR) is elected to take over if the DR fails.

If the network type of an interface is broadcast, the default OSPF priority is 1. When OSPF priorities are the same, the OSPF election for DR is decided on the router ID. The highest router ID is selected.

The election result can be determined by ensuring that the ballots, the hello packets, contain a priority for that router interface. The interface reporting the highest priority for a router will ensure that it becomes the DR.

The priorities can be set to any value from 0 to 255. A value of 0 prevents that router from being elected. A router with the highest OSPF priority will be selected as the DR. A router with the second highest priority will be the BDR. After the election process, the DR and BDR retain their roles even if routers are added to the network with higher OSPF priority values.

Router(config-if)#ip ospf priority number