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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router initializes OSPFv2 Process 1 with the following operational interfaces:
- Loopback0: 10.1.1.1/32 (status up, line protocol up)
- GigabitEthernet0/0: 172.16.10.1/24 (status up, line protocol up)

After OSPF Process 1 is fully active, a network engineer performs the following configuration actions in exact chronological sequence:
1. Configures Loopback1 with IP address 10.254.254.254/32 (status up, line protocol up).
2. Enters OSPF router configuration mode and executes the command `router-id 192.168.99.1`.
3. Configures GigabitEthernet0/1 with IP address 192.168.200.1/24 (status up, line protocol up).

Assuming no router reboot or `clear ip ospf process` command has been executed, what IP address is currently used as the operational OSPFv2 Router ID for Process 1?

Answer: 10.1.1.1

Answer

10.1.1.1
When OSPF Process 1 initially started, 10.1.1.1 was elected as the Router ID because Loopback0 was the sole operational loopback interface. OSPF Router ID selection is non-preemptive once an active process has selected its ID. Adding a loopback with a higher IP address (10.254.254.254), configuring an explicit `router-id 192.168.99.1` command, or activating new physical interfaces will not modify the operational Router ID until `clear ip ospf process` is executed or the router reboots. Thus, the active Router ID remains 10.1.1.1.

Step-by-Step Solution

1
Determine the initial OSPFv2 Router ID selected when Process 1 started.
Loopback0 (10.1.1.1) was selected because operational loopback interface IP addresses take priority over physical interface IP addresses during initial Router ID election.
In Cisco IOS OSPFv2, if no manual router ID is configured at startup, the router chooses the highest IP address among operational loopback interfaces.
2
Evaluate the impact of configuring Loopback1 (10.254.254.254/32).
The active Router ID remains 10.1.1.1.
OSPF Router ID election is non-preemptive. Dynamically elected Router IDs do not change automatically when new interfaces with higher IP addresses come online.
3
Evaluate the impact of executing `router-id 192.168.99.1`.
The active Router ID remains 10.1.1.1.
Configuring a manual OSPF router ID on an already running OSPF process does not take effect until the process is reset or the router is rebooted.
4
Evaluate the impact of adding GigabitEthernet0/1 and determine the final operational Router ID.
The active Router ID stays 10.1.1.1.
Without a process reset via `clear ip ospf process` or a device reload, none of the subsequent configuration changes displace the active Router ID.

Key Concept

OSPFv2 Router ID Non-Preemption and Reset Requirement
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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