Router Identification

Every router running OSPF within a network must have a unique router ID (RID). This identification is a 32-bit number that identifies one router to another router within an AS. The RID is used by the OSPF link-state database (LSDB) as a method of tracking each router within the AS and the links that are associated with it.

This identification number is unique to each OSPF router. You can use several methods to determine how your network decides upon the OSPF RID.

To determine the RID, OSPF uses the following process:

Step 1 Has the RID been manually set using the ospf router-id command? If not proceed to Step 2.

Step 2 If loopback interfaces are present, the highest loopback IP address in use is used. If only a single loopback address is in use, this IP address is used. If a loopback interface is not present, go to next step.

Step 3 Set the RID as the highest IP address on one of the router's active interfaces.

Configuring a loopback address on the Cisco router has the added benefit of being much more stable than the default method of using the highest IP address present on the router, because a loopback address cannot go down or lose connectivity, which would result in the need to update routing tables. The interface is essentially a software-based interface that can be used for many additional purposes, such as summarizing IP address ranges or troubleshooting. Loopback addresses are reachable, provided they fall within the advertised IP address category.

TIP When configuring the IP address for your loopback interface, remember that a "real" IP address uses valuable address space. The alternative is to use an unregistered or private IP address, which is essentially a made-up IP address that is not part of your network's normal IP address range. Refer to RFC 1918 if you decide to use this method.