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Difficulty: Very hardOSPFv2 Network Types and DR/BDR Selection

An engineer inspects the following OSPFv2 neighbor table on router R1, which is connected to a shared Layer 2 Ethernet segment:

text
R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/DROTHER 00:00:34 192.168.1.2 GigabitEthernet0/0
3.3.3.3 2 FULL/BDR 00:00:38 192.168.1.3 GigabitEthernet0/0
4.4.4.4 255 FULL/DR 00:00:31 192.168.1.4 GigabitEthernet0/0

Router R1's GigabitEthernet0/0 interface is configured with an OSPF priority of 10 and IP address 192.168.1.1. If the current Designated Router (R4) unexpectedly reboots, which router will be selected as the new Backup Designated Router (BDR) after OSPF reconverges?

  1. R1Answer
  2. B
    R2
  3. C
    R4
  4. D
    R3

Answer

Router R1 will become the new Backup Designated Router (BDR).
When the Designated Router (R4) reboots, OSPF rules mandate that the existing Backup Designated Router (R3) is immediately promoted to Designated Router (DR). A new election is then conducted to fill the vacant BDR position. Router R2 has an OSPF interface priority of 0, which completely excludes it from DR/BDR eligibility. Consequently, R1—configured with a priority of 10—is the highest-priority eligible candidate and is elected as the new BDR. Furthermore, when R4 completes its reboot, it will not preempt R3 or R1 because OSPF DR/BDR elections are non-preemptive.

Step-by-Step Solution

1
Determine the immediate role transition following DR failure.
Router R4 (the current DR) fails. In accordance with OSPF non-preemptive election mechanics, the active BDR (R3 with Router ID 3.3.3.3) is automatically promoted to DR.
OSPF ensures network stability on multi-access segments by promoting the current BDR to DR immediately when the DR goes offline.
2
Identify candidate routers eligible for the newly opened BDR role.
Active candidate routers remaining on the segment are R1 (Priority 10, RID 1.1.1.1) and R2 (Priority 0, RID 2.2.2.2).
R4 is offline rebooting, and R3 has already transitioned to DR.
3
Evaluate candidate eligibility and OSPF priority values.
R2 has an OSPF priority of 0, making it entirely ineligible for DR/BDR roles regardless of its Router ID. R1 has a valid OSPF priority of 10.
An OSPF interface priority of 0 prevents a router from ever participating in DR or BDR elections.
4
Conclude the BDR election result.
R1 is elected as the new BDR.
R1 is the sole eligible candidate remaining on the multi-access segment with an OSPF priority greater than 0.

Key Concept

OSPF DR/BDR election precedence, priority 0 ineligibility, and non-preemptive role promotion mechanics.
Estimated Time:2m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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