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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

An enterprise network has four routers—R1, R2, R3, and R4—connected to a shared Layer 2 Ethernet switch on the 10.10.10.0/24 subnet operating with default OSPFv2 configurations on a broadcast network type. Currently, R1 is functioning as the Designated Router (DR) with an interface priority of 1 and Router ID 1.1.1.1. R2 is functioning as the Backup Designated Router (BDR) with an interface priority of 1 and Router ID 2.2.2.2. R3 has its interface priority set to 0 and Router ID 3.3.3.3.

A network administrator brings up a fifth router, R5, on the same Ethernet segment with a Router ID of 5.5.5.5 and executes the following commands:

text
R5(config)# interface GigabitEthernet0/0
R5(config-if)# ip ospf priority 255

After R5 establishes FULL adjacencies with the existing routers, which two statements correctly describe the resulting OSPF DR/BDR election state and operation on this segment? (Select TWO.)

  1. R1 remains the Designated Router (DR) and R2 remains the Backup Designated Router (BDR) despite R5 having a higher priority.Answer
  2. B
    R5 immediately preempts R1 as DR because an interface priority of 255 takes immediate precedence over active routers with a priority of 1.
  3. R5 will become the DR only if both R1 and R2 fail or have their OSPF processes restarted.Answer
  4. D
    R3 will be elected as BDR if R2 fails because R3 was added to the network segment prior to R5.
  5. E
    R5 automatically converts the multiaccess segment from Broadcast to Point-to-Point mode because its priority is set to 255.

Answer

R1 remains the DR and R2 remains the BDR due to OSPF non-preemption rules, and R5 will only assume the DR role if both current DR and BDR roles are vacated.
OSPFv2 DR and BDR elections on broadcast multiaccess networks are non-preemptive. Once R1 and R2 are elected as DR and BDR respectively, introducing a new router with a higher interface priority (R5 with priority 255) will not disrupt active roles. R5 will operate as a DROTHER until a role vacancy occurs. If R1 fails, R2 is promoted to DR, and R5 would then be elected BDR. Thus, R5 can only become DR if both active DR and BDR roles are cleared.

Step-by-Step Solution

1
Evaluate the current DR/BDR roles and preemption rules in OSPFv2.
R1 is active DR and R2 is active BDR. OSPF DR/BDR elections are non-preemptive, meaning an existing DR or BDR will not be unseated by a newly introduced router even if the new router has a higher priority (such as 255).
Non-preemption prevents network instability and topology recalculations on multiaccess segments whenever a router reboots or joins.
2
Determine the effect of adding R5 with priority 255 to the segment.
R5 joins as a DROTHER initially because both DR and BDR roles are already occupied.
Elections for DR/BDR only occur when an existing DR or BDR role becomes vacant.
3
Analyze how role transition would occur upon failure of existing routers.
If R1 (DR) fails, R2 (BDR) is immediately promoted to DR. A new election occurs for BDR, which R5 will win due to its priority of 255. For R5 to become DR directly, both R1 and R2 must fail or be reset.
BDR promotion occurs before a new BDR is elected.

Key Concept

OSPFv2 DR/BDR Election Non-Preemption and Priority 0 Rules
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Topics

OSPFv2 Network Types and DR/BDR Selection

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