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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Router R3 has OSPFv2 configured and active. When the OSPF process initialized, the operational statuses of its interfaces were as follows:

- GigabitEthernet0/0 (IP: 192.168.10.1/24): Up / Line Protocol: Up
- GigabitEthernet0/1 (IP: 10.254.254.1/24): Up / Line Protocol: Up
- Loopback10 (IP: 172.16.1.1/24): Up / Line Protocol: Down
- Loopback20 (IP: 172.16.2.1/24): Up / Line Protocol: Up
- Loopback30 (IP: 172.16.3.1/24): Administratively Down / Line Protocol: Down

Shortly after initialization, a network administrator configured the command `router-id 1.1.1.1` under `router ospf 1`. No additional commands or resets were executed on the router.

What is the active OSPFv2 Router ID currently used by Router R3?

Answer: 172.16.2.1

Answer

172.16.2.1
The active OSPF Router ID is 172.16.2.1. OSPF selects a Router ID during process initialization using the following tie-breaker order: (1) an explicitly configured `router-id` command, (2) the highest IP address of any operationally active (Up/Up) loopback interface, and (3) the highest IP address of any operationally active physical interface. At initialization, Loopback20 (172.16.2.1) was the only up/up loopback interface. Although `router-id 1.1.1.1` was subsequently configured, OSPF Router ID updates are non-preemptive and require executing `clear ip ospf process` to take effect.

Step-by-Step Solution

1
Evaluate interface operational states for OSPF Router ID candidacy.
Only interfaces in an 'Up / Line Protocol: Up' state are eligible. Loopback10 (line protocol down) and Loopback20 (up/up) are evaluated. Loopback20 is the only active loopback interface.
OSPF requires an interface to be operationally active to use its IP address as a Router ID.
2
Determine the initial active OSPF Router ID during process startup.
Because no explicit `router-id` command was configured at startup, OSPF selected the highest IP address among active loopback interfaces, which is 172.16.2.1 (Loopback20).
Active loopbacks take precedence over physical interfaces regardless of IP address values.
3
Evaluate the effect of configuring `router-id 1.1.1.1` after process initialization.
The active OSPF Router ID remains 172.16.2.1.
OSPF Router ID election is non-preemptive. Changing the `router-id` CLI setting takes effect only after reloading the device or resetting the OSPF process using the `clear ip ospf process` command.

Key Concept

OSPFv2 Router ID Election Order and Non-Preemptive Behavior
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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