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Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

Five Cisco routers—R1, R2, R3, R4, and R5—are connected to a single Layer 2 Ethernet switch on the 10.1.1.0/24 multiaccess network segment running OSPFv2 in Area 0.

The initial boot sequence and interface configurations are as follows:
- R1 boots first with interface OSPF priority 1 and Router ID 1.1.1.1.
- R2 boots second with interface OSPF priority 1 and Router ID 2.2.2.2.
- Once R1 and R2 finish establishing adjacencies, R3 (priority 2, Router ID 3.3.3.3), R4 (priority 0, Router ID 4.4.4.4), and R5 (priority 255, Router ID 5.5.5.5) are booted simultaneously.

After all adjacencies reach the FULL state across the segment, R1 suffers a power failure and goes offline. Assuming no OSPF processes were reset manually prior to the failure, which router will be promoted to DR and which router will be elected as the new BDR?

  1. R2 becomes the DR, and R5 becomes the new BDR.Answer
  2. B
    R5 becomes the DR, and R3 becomes the new BDR.
  3. C
    R2 becomes the DR, and R4 becomes the new BDR.
  4. D
    R3 becomes the DR, and R5 becomes the new BDR.

Answer

R2 becomes the designated router (DR), and R5 becomes the backup designated router (BDR).
In OSPF multiaccess networks, elections are strictly non-preemptive. R1 and R2 initially take the DR and BDR roles. When R3, R4, and R5 boot later, they remain DROTHERs. When R1 fails, the active BDR (R2) is automatically promoted to DR. A new election for BDR is then conducted among the remaining DROTHERs (R3, R4, and R5). R4 is ineligible because its priority is set to 0. Between R3 (priority 2) and R5 (priority 255), R5 wins the BDR election due to having the highest interface priority.

Step-by-Step Solution

1
Determine initial DR and BDR assignments during boot sequence.
R1 boots first and becomes DR. R2 boots second and becomes BDR. R3, R4, and R5 join later and become DROTHERs despite having higher priorities because OSPF elections are non-preemptive.
OSPF DR/BDR election occurs as routers come online. Once DR and BDR are elected, newly attached routers with higher priorities will not displace existing DR/BDR roles.
2
Analyze what happens to the DR role when R1 (DR) fails.
R2 (the existing BDR) is immediately promoted to DR.
Upon DR failure, the existing BDR always steps up to become the new DR to prevent election delays and instability.
3
Evaluate the new BDR election among remaining routers (R3, R4, R5).
R4 has priority 0 and is ineligible. Between R3 (priority 2) and R5 (priority 255), R5 has the highest priority and is elected as the new BDR.
An interface priority of 0 (ip ospf priority 0) disqualifies a router from DR/BDR election. Among eligible routers, highest priority wins.

Key Concept

OSPF Non-Preemptive DR/BDR Election Mechanics and Priority Rules
Estimated Time:2m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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