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Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

An enterprise network has a shared Ethernet segment running OSPFv2 in Area 0. Router R1 (Router ID 10.1.1.1, priority 1) is currently serving as the Designated Router (DR), and Router R2 (Router ID 10.2.2.2, priority 1) is serving as the Backup Designated Router (BDR). A network engineer adds Router R5 (Router ID 10.5.5.5, priority 255) to the segment. Shortly after R5 stabilizes as a DROTHER, R1's interface experiences a temporary link failure and goes down for five minutes before recovering. Which router functions as the DR on the segment immediately after R1 recovers and reaches the FULL neighbor state?

  1. Router R2 functions as the DR because it was promoted from BDR when R1 failed, and OSPF DR elections are non-preemptive.Answer
  2. B
    Router R5 functions as the DR because its priority of 255 is higher than any other router on the segment.
  3. C
    Router R1 functions as the DR because it reclaims its original role upon completing adjacency initialization with R2 and R5.
  4. D
    Router R5 functions as the DR and Router R1 functions as the BDR because R1 has a higher Router ID than R2.

Answer

Router R2 remains the Designated Router (DR) because OSPF DR/BDR elections are non-preemptive. Upon R1's failure, R2 was promoted to DR and R5 became BDR; when R1 recovers, existing roles are maintained.
OSPF DR and BDR roles are non-preemptive. When R5 joined initially, R1 and R2 retained their roles despite R5 having a higher priority. When R1 failed, BDR R2 was promoted to DR, and R5 was elected as the new BDR. When R1 recovers, it finds an active DR (R2) and an active BDR (R5) already present on the segment, so R1 becomes a DROTHER.

Step-by-Step Solution

1
Analyze initial state and addition of R5
R1 is DR, R2 is BDR. Adding R5 with priority 255 does not trigger a re-election for DR/BDR because active roles are non-preemptive. R5 becomes a DROTHER.
OSPF avoids network instability by preventing newly connected routers from preempting active DR and BDR roles.
2
Evaluate state changes during R1 failure
When R1 fails, BDR R2 is immediately promoted to DR. An election is held to fill the vacant BDR role, which R5 wins due to having the highest priority (255).
The BDR automatically steps up to DR when the DR fails, and remaining eligible routers elect a new BDR.
3
Determine final state after R1 link recovery
R1 re-establishes adjacency, sees active DR (R2) and active BDR (R5), and assumes the DROTHER role.
Recovered or newly booted routers do not preempt existing operational DR/BDR routers on multiaccess networks.

Key Concept

Non-preemptive nature of OSPF DR/BDR elections and promotion mechanics on broadcast multiaccess networks
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Topics

OSPFv2 Network Types and DR/BDR Selection

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