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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Place the criteria evaluated during an OSPFv2 Designated Router (DR) election in order of precedence, starting with the highest priority evaluation criteria first.

  1. 1Highest interface OSPF priority value (non-zero)
  2. 2Highest manually configured OSPF Router ID
  3. 3Highest IP address among active loopback interfaces
  4. 4Highest IP address among active physical interfaces

Answer

The correct order of precedence for OSPFv2 DR/BDR election tie-breaking is: 1. Highest interface OSPF priority value (non-zero), 2. Highest manually configured OSPF Router ID, 3. Highest IP address among active loopback interfaces, 4. Highest IP address among active physical interfaces.
In OSPFv2 DR/BDR elections on broadcast or non-broadcast multiaccess networks, the selection process follows a strict precedence: interface priority is checked first (highest wins, while priority 0 is ineligible). If priorities tie, the OSPF Router ID determines the winner. The Router ID selection itself follows a deterministic hierarchy: explicit manual configuration via 'router-id' first, followed by the highest active loopback IP address, and finally the highest active physical interface IP address.

Step-by-Step Solution

1
Evaluate OSPF Interface Priority
Highest priority (1-255) wins. A priority of 0 excludes the router from becoming DR/BDR.
OSPF prioritizes administrator design intent via interface priority over IP address tie-breakers.
2
Evaluate Explicit Router ID
If priority values are equal, the router with the highest manually configured router-id value is selected.
Manual Router ID configuration overrides dynamically determined interface IP addresses.
3
Evaluate Active Loopback Interfaces
If no manual router ID exists, the highest IP address configured on an active loopback interface is chosen.
Loopback interfaces are logically stable and preferred over physical interfaces for RID determination.
4
Evaluate Active Physical Interfaces
If no loopback interfaces exist, the highest IP address configured on an active physical interface is used as the final tie-breaker.
Physical interface IPs serve as the final fallback for Router ID determination in OSPFv2.

Key Concept

OSPFv2 DR/BDR Election Precedence and Router ID Selection Criteria
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Topics

OSPFv2 Network Types and DR/BDR Selection

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