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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network engineer initializes OSPFv2 on a Cisco router using the configuration below:

router ospf 10
router-id 1.1.1.1
network 10.1.1.0 0.0.0.3 area 0

The router also has an active Loopback 0 interface with IP address 172.16.1.1/24 and an active GigabitEthernet0/0 interface with IP address 10.1.1.1/30.

After neighbor adjacencies are established, the engineer enters OSPF configuration mode and executes the command `router-id 2.2.2.2`. No further commands are issued, and the router is not rebooted.

Which IP address is currently used as the active OSPF Router ID by the running OSPF process?

  1. 1.1.1.1Answer
  2. B
    2.2.2.2
  3. C
    172.16.1.1
  4. D
    10.1.1.1

Answer

1.1.1.1 is the active OSPF Router ID because changing the router-id configuration does not update the operational Router ID until the OSPF process is cleared or the router is restarted.
The correct choice is 1.1.1.1. When OSPF initializes, the explicit `router-id` command overrides all interface IP addresses. Once selected and active, changing the `router-id` setting does not dynamically change the operational router ID until the OSPF process is restarted using the `clear ip ospf process` privileged EXEC mode command or by reloading the device.

Step-by-Step Solution

1
Determine initial Router ID election
The OSPF process elected 1.1.1.1 because an explicit `router-id` command takes highest precedence during initialization.
Explicit configuration overrides loopback and physical interface IP addresses.
2
Analyze the impact of changing the `router-id` command on an active process
Configuring `router-id 2.2.2.2` updates the running configuration file, but does NOT dynamically reset active neighbor adjacencies or change the live operational Router ID.
OSPF requires an explicit process reset via `clear ip ospf process` or a device reboot to enact a new Router ID.
3
Identify the current operational Router ID
The operational Router ID remains 1.1.1.1.
No process restart command was executed after changing the configuration.

Key Concept

OSPFv2 Router ID Non-Preemptive Precedence and Process Reset Requirement
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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