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Difficulty: Very hardOSPFv2 Network Types and DR/BDR Selection

Four OSPFv2 routers are connected to a shared Layer 2 Ethernet switch on the 192.168.10.0/24 broadcast multiaccess network in Area 0. The interface configurations and boot order are as follows:

1. First, Router R3 (Priority 1, Router ID 10.3.3.3) and Router R4 (Priority 2, Router ID 10.1.1.1) booted up and established an adjacency. R4 was elected as the Designated Router (DR) and R3 was elected as the Backup Designated Router (BDR).
2. Later, Router R1 (Priority 0, Router ID 10.10.10.10) and Router R2 (Priority 2, Router ID 10.2.2.2) joined the segment and established adjacencies with all routers.

If Router R4 suddenly suffers a power outage and drops offline, which routers will assume the roles of Designated Router (DR) and Backup Designated Router (BDR) on this network segment?

  1. Router R3 becomes the DR, and Router R2 becomes the BDR.Answer
  2. B
    Router R2 becomes the DR, and Router R3 remains the BDR.
  3. C
    Router R3 becomes the DR, and Router R1 becomes the BDR.
  4. D
    Router R1 becomes the DR, and Router R2 becomes the BDR.

Answer

Router R3 becomes the Designated Router (DR), and Router R2 becomes the Backup Designated Router (BDR).
When the active Designated Router (R4) fails, OSPF non-preemption rules mandate that the existing Backup Designated Router (R3) is automatically promoted to DR. An election is then held solely for the newly vacated BDR position. Between the remaining routers, R1 is ineligible due to its OSPF priority of 0, leaving R2 (Priority 2) as the sole eligible router to become BDR.

Step-by-Step Solution

1
Evaluate initial state prior to failure.
R4 is DR (Priority 2) and R3 is BDR (Priority 1). When R1 (Priority 0) and R2 (Priority 2) join later, no preemption occurs. R4 remains DR and R3 remains BDR.
OSPF DR and BDR roles are non-preemptive once elected on a broadcast multiaccess segment.
2
Determine DR replacement when R4 fails.
R3 (the active BDR) immediately transitions to DR.
When an active DR fails, the existing BDR is automatically promoted to DR without triggering a brand new DR election from scratch.
3
Evaluate candidates for the vacant BDR role among remaining active routers (R1 and R2).
R1 is disqualified because its priority is 0 (`ip ospf priority 0`). R2 has priority 2 (> 0) and is eligible.
Routers with an OSPF priority of 0 are permanently excluded from DR/BDR elections and remain in DROTHER state.
4
Conclude final roles.
R3 is DR and R2 is elected as the new BDR.
R2 is the only remaining eligible router with a priority greater than 0.

Key Concept

OSPF DR/BDR election rules: Non-preemption, BDR promotion, and Priority 0 exclusion.
Estimated Time:2m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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