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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

An engineer inspects the OSPFv2 neighbor status on router R1, which is connected to a shared Ethernet segment alongside routers R2, R3, and R4. The command output on R1 displays the following neighbor table:

text
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/DR 00:00:33 192.168.1.2 GigabitEthernet0/0
3.3.3.3 1 FULL/BDR 00:00:36 192.168.1.3 GigabitEthernet0/0
4.4.4.4 255 FULL/DROTHER 00:00:31 192.168.1.4 GigabitEthernet0/0

The administrator wants R4 (Router ID 4.4.4.4) to become the Designated Router (DR) on this network segment without changing any interface priorities. Which action will force a new DR election across the segment?

  1. Execute the clear ip ospf process command on all routers on the multiaccess segment.Answer
  2. B
    Reload router R4 so that its interface priority of 255 forces an immediate preemption of the active DR.
  3. C
    Configure ip ospf priority 0 on router R1's GigabitEthernet0/0 interface.
  4. D
    Change the OSPF network type on interface GigabitEthernet0/0 on R4 to point-to-point.

Answer

Execute the clear ip ospf process command on all routers on the multiaccess segment.
The correct answer identifies that OSPFv2 DR/BDR elections are non-preemptive. Once a DR and BDR are elected on a multiaccess broadcast network, a newly added router with a higher interface priority (such as R4 with priority 255) will remain in the DROTHER state. To force a new election where R4 can claim the DR role based on its priority, the OSPF adjacencies across the segment must be restarted using the `clear ip ospf process` command.

Step-by-Step Solution

1
Analyze the current DR/BDR operational roles and OSPF priorities from the output snippet.
R2 (priority 1) is currently the active DR, R3 (priority 1) is the BDR, and R4 (priority 255) is in the DROTHER state.
R4 likely joined the segment after R2 and R3 had already established DR and BDR roles.
2
Evaluate OSPFv2 preemption behavior.
OSPF DR/BDR elections are non-preemptive. Even though R4 has a higher interface priority (255>1255 > 1), it will not displace an active DR or BDR while their OSPF processes remain operational.
Non-preemption prevents network instability and frequent routing table recalculations.
3
Determine the necessary operational command to trigger a fresh election.
Executing `clear ip ospf process` forces all routers on the broadcast domain to reset their OSPF adjacencies and conduct a new DR/BDR election.
During a new election with all routers participating simultaneously, R4 will win the DR role because it has the highest configured interface priority.

Key Concept

OSPFv2 DR/BDR elections on broadcast multiaccess networks are non-preemptive; active DR/BDR roles persist until adjacencies are reset or the active DR/BDR interfaces go down.
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Topics

OSPFv2 Network Types and DR/BDR Selection

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