OSPF DR/BDR Election Rules and Preemption Behavior
Short answer
BD Zero priority prevents adjacency, and larger DR priority values indicate higher election priority.
Question
Key Takeaways
The main trap is confusing the tie-breaker rule (highest Router ID wins) and assuming DR election is preemptive like VRRP.
Understand OSPF DR/BDR election mechanics on MA networks, including how zero priority affects adjacency and why larger priority values win.
Common mistake: Choosing A because candidates often mistakenly believe smaller Router IDs are preferred, or choosing C due to confusion with other protocols like HSRP/VRRP.
Expert Analysis
Correct Answer: B, D
DR/BDR election on OSPF broadcast networks is designed to reduce adjacency overhead by selecting designated routers. The selection process follows a strict hierarchy based on interface configuration and router identity.
Option B is correct. When an OSPF interface priority is set to 0, the router is explicitly disqualified from participating in the DR or BDR election. If two routers connect via an interface where both have a priority of 0, neither can become the DR or BDR, and they cannot form a full adjacency (they will remain in the Exchange/Loading state indefinitely). This is a critical operational constraint for ensuring network stability.
Option D is correct. The primary determinant for DR/BDR election is the OSPF interface priority. A higher priority value (ranging from 0 to 255) indicates a higher likelihood of being elected. By default, the priority is 1. This setting allows network administrators to influence which router becomes the DR based on hardware capability or link reliability.
Incorrect Options Analysis:
- Option A is incorrect. When priorities are equal, the Router ID is used as the tie-breaker. However, the rule states that the router with the largest (highest) Router ID wins the election, not the smallest. Selecting the smallest Router ID would lead to incorrect topology formation.
- Option C is incorrect. OSPF DR/BDR election is non-preemptive. Once a DR or BDR is elected, it retains its role until it fails or the interface goes down, even if a new router with a higher priority joins the network later. This prevents unnecessary flooding of LSAs and topology changes caused by frequent elections.
This knowledge covers the nuances of OSPF neighbor states, specifically how non-zero vs. zero priorities affect adjacency formation, and why preemption is disabled to maintain routing stability.
Exam Strategy
Always check the interface priority first; remember that 0 means 'no election'. Second, verify if the question implies preemption—OSPF never preempts. Finally, recall that tie-breakers favor the highest Router ID, unlike some other systems that use lowest values.
Frequently Asked Questions
What happens if both routers on a segment have OSPF priority 0?
Is OSPF DR election preemptive?
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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HCIP Resolutor 👍 1A es FALSA porque a mayor valor de Router ID mayor es la prioridad (no menor). Por lo tanto, las únicas respuestas verdaderas son B y D. A continuación se desglosa el análisis técnico de cada opción.
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