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Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

Four Cisco routers (R1, R2, R3, and R4) are connected to a shared Ethernet broadcast segment (10.1.1.0/24). OSPFv2 process 1 is enabled simultaneously on all router interfaces attached to this segment. The relevant interface and router parameters are configured as follows:

- R1: Interface OSPF Priority = 100, Router ID = 192.168.1.1
- R2: Interface OSPF Priority = 200, Router ID = 10.2.2.2
- R3: Interface OSPF Priority = 200, Router ID = 10.3.3.3
- R4: Interface OSPF Priority = 0, Router ID = 10.254.254.254

Assuming all routers initialize at the same moment without any existing DR or BDR active on the link, which router will be elected as the Backup Designated Router (BDR)?

  1. R2Answer
  2. B
    R3
  3. C
    R4
  4. D
    R1

Answer

R2 will be elected as the Backup Designated Router (BDR).
In OSPF multiaccess networks, DR and BDR elections evaluate interface priority first. Routers with priority 0 (R4) are excluded. Between R2 and R3 (both priority 200), R3 wins the DR role due to its higher Router ID (10.3.3.3 > 10.2.2.2). R2 has the highest remaining priority among eligible non-DR routers, making it the BDR.

Step-by-Step Solution

1
Filter out ineligible routers based on OSPF interface priority.
R4 has an interface priority of 0 and is excluded from both DR and BDR elections.
A priority value of 0 explicitly prevents a router from becoming a DR or BDR.
2
Compare the interface priority values of the remaining eligible routers (R1, R2, R3).
R2 and R3 tie for the highest priority (200), while R1 has a lower priority (100).
OSPF elects DR and BDR based primarily on the highest interface priority.
3
Break the tie between R2 and R3 to elect the Designated Router (DR).
R3's Router ID (10.3.3.3) is higher than R2's Router ID (10.2.2.2), so R3 becomes DR.
When priority values are identical, the router with the highest Router ID wins the election.
4
Select the Backup Designated Router (BDR) from the remaining eligible routers.
R2 becomes the BDR because it has the highest remaining priority (200).
After DR selection, the router with the highest remaining priority (or highest Router ID in case of another tie) is selected as BDR.

Key Concept

OSPFv2 DR/BDR Election Rules and Priority 0 Ineligibility
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Topics

OSPFv2 Network Types and DR/BDR Selection

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