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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network engineer configures an OSPFv2 routing process on router R1. At startup, the router has two operational interfaces: Loopback0 configured with IP address 10.1.1.1/3210.1.1.1/32 and GigabitEthernet0/0 configured with IP address 172.16.1.1/24172.16.1.1/24. The OSPF process initializes and elects 10.1.1.110.1.1.1 as its Router ID. Later, the engineer configures a new interface, Loopback1, with IP address 10.200.1.1/3210.200.1.1/32. Upon issuing the `show ip ospf` command, the engineer observes that the active OSPF Router ID remains 10.1.1.110.1.1.1. Which action must be taken for R1 to begin using 10.200.1.110.200.1.1 as its active OSPF Router ID?

  1. Execute the clear ip ospf process command in privileged EXEC mode.Answer
  2. B
    Configure the ip ospf priority 255 command under interface Loopback1.
  3. C
    Change the OSPF Area ID on interface Loopback1 to Area 0.
  4. D
    Shut down interface GigabitEthernet0/0 to force OSPF to re-evaluate active interfaces.

Answer

The network engineer must execute the 'clear ip ospf process' command in privileged EXEC mode to restart the OSPF routing process and trigger a re-election of the Router ID.
OSPF Router ID election is non-preemptive. Once the OSPF process selects a Router ID during initialization, it retains that ID even if a new interface with a higher IP address becomes active. To force the router to evaluate interfaces again and select the newly configured higher loopback address, the administrator must reset the routing process using the privileged EXEC command 'clear ip ospf process'.

Step-by-Step Solution

1
Understand OSPF Router ID selection priority.
OSPF Router ID is selected based on: 1) Manually configured 'router-id' command, 2) Highest IP address among active loopback interfaces, 3) Highest IP address among active non-loopback interfaces.
Loopback interfaces are preferred over physical interfaces due to their stability.
2
Analyze the non-preemptive nature of the OSPF Router ID.
Once an OSPF Router ID is elected and the OSPF process is running, adding a new interface with a higher IP address will NOT dynamically change the active Router ID.
Changing the Router ID dynamically would reset all neighbor adjacencies, so OSPF maintains stability by keeping the current Router ID.
3
Determine the required action to apply the new Router ID.
To force OSPF to adopt the new highest active loopback IP address (10.200.1.1), the OSPF process must be restarted using 'clear ip ospf process' or by reloading the device.
Restarting the OSPF process re-initializes election parameters and picks the new highest loopback address.

Key Concept

OSPFv2 Router ID Election and Non-Preemptive Behavior
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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