Cisco CCNA (200-301)IP ConnectivityMedium
A network administrator is configuring OSPFv2 on a router. The router has three active interfaces: Loopback0 (IP 192.168.1.1/32), GigabitEthernet0/0 (IP 10.0.0.1/24), and GigabitEthernet0/1 (IP 172.16.0.1/24). All interfaces are configured under OSPF area 0. The administrator has NOT manually configured an OSPF router ID. Which IP address will the router select as its OSPF Router ID?
- A192.168.1.1
- B172.16.0.1
- CThe highest IP address of any active interface, which is 192.168.1.1.
- D10.0.0.1
Show answer & explanationAnswer & explanation
Correct answer: A. 192.168.1.1
OSPF Router ID selection prioritizes loopback interfaces. If multiple loopback interfaces exist, the router selects the highest IP address among them. If no loopback interfaces are configured, it selects the highest IP address among active physical interfaces. In this scenario, 192.168.1.1 is the only loopback interface and will be chosen as the Router ID.
Why the other options are wrong
- B. This is a physical interface IP, which is considered after loopbacks.
- C. While it is the highest IP, the *reason* it's chosen is its loopback status, not just being the highest among all interfaces. The statement is partially correct but the reasoning is incomplete/misleading regarding the order of preference.
- D. This is a physical interface IP, which is considered after loopbacks.
OSPF Router ID Selection
OSPF Router ID is a 32-bit value unique to each router, chosen by highest IP on active loopback, then highest IP on active physical interface, unless manually configured.
- Manual configuration (router-id command) takes highest precedence.
- If no manual configuration, highest IP of an active loopback interface is chosen.
- If no loopback interfaces, highest IP of an active physical interface is chosen.
- Router ID selection happens at OSPF process startup.
Memory trick: Manual is king, Loopback is queen, Physical is jack, to find the ID scene.