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Difficulty: Very hardOSPFv2 Network Types and DR/BDR Selection

An enterprise network administrator deploys four Cisco routers (R1, R2, R3, and R4) connected to a common Layer 2 GigabitEthernet switch on the 192.168.1.0/24192.168.1.0/24 broadcast multiaccess segment. All routers are powered on simultaneously, and OSPFv2 Area 0 is initialized across all connected interfaces. The interface parameters and operational configurations are as follows:

* R1: Configured with `ip ospf priority 255` and `ip ospf network point-to-point`; Router ID is 1.1.1.11.1.1.1.
* R2: Configured with default interface priority (11); Router ID is 10.10.10.1010.10.10.10 (derived from Loopback0).
* R3: Configured with `ip ospf priority 0`; Router ID is 30.30.30.3030.30.30.30 (derived from Loopback0).
* R4: Configured with interface priority set to 100100 (`ip ospf priority 100`); Router ID is 20.20.20.2020.20.20.20 (derived from Loopback0).

Assuming all routers run default OSPF timers and IP settings match, which two outcomes correctly describe the resulting OSPFv2 operational states and DR/BDR election results on this segment? (Choose two.)

  1. R4 is elected as the Designated Router (DR) for the broadcast multiaccess segment.Answer
  2. B
    R1 is elected as the Designated Router (DR) because it has the highest interface priority of 255.
  3. R2 is elected as the Backup Designated Router (BDR) for the broadcast multiaccess segment.Answer
  4. D
    R3 is elected as the Backup Designated Router (BDR) because its Router ID (30.30.30.30) is the highest among all participating routers.
  5. E
    R1 successfully establishes a FULL neighbor adjacency with R4 because their Hello and Dead timers naturally match.

Answer

The statement identifying R4 as the DR and the statement identifying R2 as the BDR are the correct choices.
In OSPFv2, DR/BDR elections only take place on multiaccess network types (such as broadcast). Because R1's interface is set to point-to-point, it does not participate in DR/BDR elections despite its high priority of 255. Furthermore, R3's priority of 0 explicitly disqualifies it from becoming DR or BDR. Comparing the remaining eligible routers on the broadcast network (R2 with priority 1 and R4 with priority 100), R4 wins the DR election due to its higher priority (100), and R2 wins the BDR election as the remaining eligible candidate.

Step-by-Step Solution

1
Evaluate eligibility for DR/BDR election based on OSPF network type
R1 is configured with network type point-to-point. Point-to-point networks do not perform DR/BDR elections. Therefore, R1 is completely eliminated from the election regardless of its priority of 255.
DR/BDR selection only occurs on multiaccess network types (Broadcast and Non-Broadcast Multi-Access).
2
Evaluate eligibility for DR/BDR election based on interface priority values
R3 has an interface priority of 0 (`ip ospf priority 0`), which renders it ineligible to become either DR or BDR. Thus, R3 is eliminated from election consideration.
An OSPF interface priority of 0 explicitly configured on an interface prevents the router from participating in DR/BDR elections.
3
Determine DR and BDR election winners among remaining eligible candidates
The remaining eligible candidates on the broadcast segment are R4 (priority 100) and R2 (priority 1). R4 has the highest priority and is elected DR. R2 has the next highest priority among eligible routers and is elected BDR.
OSPF DR election prioritizes highest interface priority first, followed by highest Router ID as a tie-breaker.

Key Concept

OSPFv2 DR/BDR Election Mechanics and Interface Network Type Rules
Estimated Time:3m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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