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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

A network engineer is managing a broadcast multiaccess Ethernet segment with four routers running OSPFv2 in Area 0. The output below is taken from router R2:

text
R2# show ip ospf interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 192.168.10.2/24, Area 0
Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
State DROTHER, Priority 0
Designated Router (ID) 4.4.4.4, Interface address 192.168.10.4
Backup Designated Router (ID) 3.3.3.3, Interface address 192.168.10.3

The engineer executes the command `ip ospf priority 250` on R2's GigabitEthernet0/0 interface. Shortly after, the current Designated Router (R4) reboots. Assuming all other routers maintain their default priority of 1 and no OSPF processes are cleared manually, which role will router R2 assume on the segment after convergence?

  1. Backup Designated Router (BDR)Answer
  2. B
    Designated Router (DR)
  3. C
    DROTHER
  4. D
    2-WAY

Answer

Backup Designated Router (BDR)
In OSPFv2 broadcast networks, elections are non-preemptive. When the active DR (R4) fails or reboots, the existing BDR (R3) is automatically promoted to DR. A new election is then held specifically to fill the now-vacant BDR role. Since R2's interface priority was increased from 0 to 250, R2 is eligible and wins the BDR election over any remaining routers with default priority 1. Therefore, R2 becomes the Backup Designated Router (BDR).

Step-by-Step Solution

1
Analyze initial OSPF roles and interface priority configuration
R4 is DR (Router ID 4.4.4.4), R3 is BDR (Router ID 3.3.3.3), and R2 is DROTHER because its priority was 0.
An OSPF priority of 0 excludes a router interface from participating in DR/BDR elections.
2
Evaluate the impact of changing interface priority to 250 on R2
R2 becomes eligible to participate in future DR/BDR elections with a priority of 250, but elections are non-preemptive while active DR/BDR roles are filled.
OSPF DR/BDR elections do not preempt active role holders upon configuration changes.
3
Determine role transitions following the failure/reboot of the DR (R4)
R3 (the active BDR) is immediately promoted to DR. A new election takes place to fill the vacant BDR position.
OSPF rules state that when a DR fails, the active BDR automatically becomes the new DR.
4
Select the winner of the new BDR election
R2 wins the election for BDR because its priority (250) is higher than the remaining candidate routers (priority 1).
The router with the highest non-zero priority wins the BDR election.

Key Concept

OSPF DR/BDR Election Non-Preemption and Promotion Rules
Estimated Time:1m 30s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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