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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Four routers (R1, R2, R3, and R4) are connected to a shared Ethernet switch in OSPFv2 Area 0. R1 (Router ID 1.1.1.1, interface priority 1) boots up first and completes initialization. Next, R2 (Router ID 2.2.2.2, interface priority 1) boots up and joins the network segment. Several minutes later, R3 (Router ID 3.3.3.3, interface priority 255) and R4 (Router ID 4.4.4.4, interface priority 0) are booted up and added to the OSPF segment. Assuming no OSPF processes are restarted or cleared, which router serves as the Designated Router (DR) on this multiaccess segment?

  1. R1Answer
  2. B
    R3
  3. C
    R2
  4. D
    R4

Answer

R1 remains the Designated Router (DR) because OSPF DR elections are non-preemptive.
OSPF DR/BDR elections are non-preemptive on broadcast multiaccess networks. Since R1 booted first and was elected Designated Router, it retains the DR role continuously regardless of new routers with higher priorities or higher Router IDs joining the network later.

Step-by-Step Solution

1
Evaluate the boot order and initial election outcome
R1 boots first and becomes the DR. R2 boots second and becomes the BDR.
When R1 boots, no existing DR exists on the segment, so R1 assumes the DR role. R2 joins next and claims the vacant BDR role.
2
Analyze the impact of adding R3 and R4 to an operational segment
R3 and R4 form adjacencies but do not trigger a new DR election.
OSPF DR/BDR elections are non-preemptive. Once a DR and BDR are active on a broadcast multiaccess segment, newly added routers with higher priority or higher Router IDs will not preempt the current DR or BDR.
3
Determine the final DR role assignment
R1 maintains its role as DR.
Because R1 has not failed or restarted its OSPF process, it continues functioning as the active DR.

Key Concept

OSPFv2 DR/BDR Non-Preemption
Estimated Time:1m 30s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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