Cisco Certified Network Associate (CCNA) Practice Exam

By default, what does an OSPF router use as its router ID?

1. Highest loopback IP address

2. Highest physical interface IP address

3. Lowest IP address on the router

4. Highest physical interface, then highest loopback interface IP address

The correct answer is: Highest physical interface, then highest loopback interface IP address

In OSPF (Open Shortest Path First), the router ID is a unique identifier for each router within a specific OSPF area. By default, OSPF selects the router ID by following a specific order of precedence. First, if any loopback interfaces are configured on the router, OSPF will select the highest IP address assigned to these loopback interfaces as the router ID. If no loopback interfaces are configured, OSPF will then consider the physical interface IP addresses. Among the physical interfaces, OSPF chooses the highest IP address assigned across those interfaces. This two-tier selection process—first looking for the highest loopback interface IP address and then falling back to the highest physical interface IP address—ensures that each router has a consistent and unique identifier. If both loopback and physical interfaces are present, the loopback interface IP is preferred due to its stability and the fact that it is always 'up' as long as the router is operational. Thus, the option that describes this sequential logic accurately conveys the manner in which OSPF determines the router ID by giving priority first to the loopback IP address and then to the highest physical interface IP address, making it the correct answer.