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Difficulty: EasyOSPFv2 Network Types and DR/BDR Selection

Four candidate routers are connected to a shared multiaccess Ethernet segment to participate in an OSPFv2 Designated Router (DR) election. Arrange the evaluation criteria in order of precedence, starting with the primary criterion checked first at the top down to the final fallback tie-breaker at the bottom.

  1. 1Highest OSPF interface priority setting
  2. 2Highest manually configured OSPF router ID
  3. 3Highest IPv4 address on an active loopback interface
  4. 4Highest IPv4 address on an active physical interface

Answer

The correct order of precedence for OSPF DR election tie-breaking is: 1) Highest OSPF interface priority setting, 2) Highest manually configured OSPF router ID, 3) Highest IPv4 address on an active loopback interface, and 4) Highest IPv4 address on an active physical interface.
The Designated Router election first evaluates the interface priority configured on the multiaccess segment. If priorities tie, OSPF resolves the election using the router's Router ID (RID). The RID is determined in order by: 1) explicit manual configuration via 'router-id', 2) highest IP address on an active loopback interface, and 3) highest IP address on an active physical interface.

Step-by-Step Solution

1
Evaluate segment interface priority
Highest interface priority wins the election (priority 0 disqualifies a router from becoming DR/BDR).
Interface priority (configured via 'ip ospf priority') is the top-level override mechanism for DR selection.
2
Check for explicit OSPF Router ID
If interface priorities tie, OSPF compares Router IDs, prioritizing a manually configured 'router-id'.
Manual Router ID configuration under the 'router ospf' process overrides dynamic interface IP selection.
3
Check active loopback interfaces
If no manual router ID is present, the highest IPv4 address on an active loopback interface is chosen as the Router ID.
Loopback interfaces offer stability over physical interfaces because they do not rely on physical link state.
4
Check active physical interfaces
If no loopback interfaces exist, the highest IPv4 address on an up/up physical interface becomes the Router ID.
This serves as the final dynamic tie-breaker for establishing the OSPF Router ID when no explicit or loopback settings exist.

Key Concept

OSPFv2 DR/BDR Election Precedence and Router ID Selection Criteria
Estimated Time:1m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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