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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Router HQ-RTR has been actively running OSPFv2 process 10 with several interfaces active. When the OSPF process was originally started, the active interfaces were Loopback0 (172.16.1.1/32), GigabitEthernet0/0 (192.168.1.1/24), and GigabitEthernet0/1 (10.10.10.1/24), with no explicitly configured router ID.

Later, a network engineer configures Loopback1 with IP address 10.255.255.255/32 and executes the command `router-id 1.1.1.1` under OSPF process 10. However, the engineer does not reload the router or restart the OSPF process.

When HQ-RTR sends Hello packets to a newly connected OSPF neighbor on GigabitEthernet0/0, which IP address is used as HQ-RTR's OSPF Router ID?

  1. 172.16.1.1, because OSPF Router ID changes are non-preemptive and require restarting the OSPF process to take effectAnswer
  2. B
    1.1.1.1, because an explicitly configured router ID immediately overrides all interface IP addresses
  3. C
    10.255.255.255, because Loopback1 has the highest IPv4 address among all active loopback interfaces
  4. D
    192.168.1.1, because GigabitEthernet0/0 has the highest IPv4 address among active physical interfaces

Answer

172.16.1.1, because OSPF Router ID changes are non-preemptive and require restarting the OSPF process to take effect
The correct answer explains that OSPF Router ID election is non-preemptive. When OSPF process 10 was first initialized, Loopback0 (172.16.1.1) was chosen as the Router ID because it had the highest IP address among active loopback interfaces. Subsequent changes—such as adding a higher loopback address or entering the `router-id` command—will take effect only after restarting the OSPF process using `clear ip ospf process` or reloading the router.

Step-by-Step Solution

1
Determine initial OSPF Router ID selection
Loopback0 (172.16.1.1) was selected as the Router ID when OSPF process 10 initially started, as it was the highest IP address on an active loopback interface.
OSPF Router ID selection order is: 1) manual `router-id` command, 2) highest IP on an active loopback interface, 3) highest IP on an active physical interface.
2
Evaluate the impact of subsequent configuration changes without process restart
Configuring Loopback1 (10.255.255.255) and setting `router-id 1.1.1.1` updates the configured state in running-config, but does NOT dynamically update the active operational Router ID.
OSPF Router ID election is non-preemptive. Changes take effect only after the router reboots or the administrator executes `clear ip ospf process`.
3
Identify the active Router ID used in outgoing Hello packets
The active Router ID remains 172.16.1.1.
Because no process restart occurred, HQ-RTR continues to identify itself using 172.16.1.1 in OSPF Hello packets.

Key Concept

OSPFv2 Router ID Precedence and Non-Preemptive Election Logic
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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