Cisco CCNA (200-301)IP ConnectivityEasy
A router has two active interfaces: Loopback0 with IP 10.0.0.1 and GigabitEthernet0/0 with IP 192.168.1.1. No router-id is manually configured under the OSPF process. Which IP address will OSPF select as the Router ID?
- AOSPF will fail to start without a manually configured router-id
- BThe lowest IP address among all interfaces, which is 10.0.0.1
- C10.0.0.1, because loopback interfaces are preferred over physical interfaces
- D192.168.1.1, because Gigabit interfaces are always preferred
Show answer & explanationAnswer & explanation
Correct answer: C. 10.0.0.1, because loopback interfaces are preferred over physical interfaces
OSPF selects the Router ID using this priority: manually configured router-id first, then the highest IP address on any active loopback interface, then the highest IP address on any active physical interface. Since a loopback exists (10.0.0.1), it is chosen regardless of being numerically lower than the physical interface.
Why the other options are wrong
- A. OSPF can start without a manual router-id by auto-selecting one from interfaces.
- B. OSPF does not pick the lowest IP; it prefers loopbacks over the highest physical IP.
- D. Interface type (Gigabit vs Fast) has no bearing on router-id selection.
OSPF Router ID Selection
OSPF chooses its Router ID in this order: manually configured router-id, highest IP on an active loopback, then highest IP on an active physical interface.
- Loopbacks always beat physical interfaces
- Router ID must be unique per OSPF domain
- Changing router-id requires clearing OSPF process or reboot
Memory trick: Loopback is loyal—it always wins the ID crown