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Difficulty: Very hardOSPFv2 Network Types and DR/BDR Selection

An enterprise network engineer adds a new router, Core-GW3, to an existing OSPFv2 broadcast multiaccess Ethernet segment (172.16.10.0/24172.16.10.0/24) where Router-A (Router ID 1.1.1.11.1.1.1, Priority 100100) is currently acting as the Designated Router (DR) and Router-B (Router ID 2.2.2.22.2.2.2, Priority 100100) is acting as the Backup Designated Router (BDR).

The engineer issues the following command on Core-GW3:

text
Core-GW3# show ip ospf interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 172.16.10.3/24, Area 0
Process ID 1, Router ID 10.10.10.10, Network Type BROADCAST, Cost: 1
Enabled by interface configuration option
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 1.1.1.1, Interface address 172.16.10.1
Backup Designated Router (ID) 2.2.2.2, Interface address 172.16.10.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

If Router-A (the active DR) suddenly experiences a hardware failure and reboots, which role will Core-GW3 assume on this segment?

  1. A
    Core-GW3 will become the new Designated Router (DR) because its Router ID (10.10.10.10) is the highest on the segment.
  2. B
    Core-GW3 will become the new Backup Designated Router (BDR) after Router-B is promoted to Designated Router.
  3. Core-GW3 will remain in the DROTHER state and will not be elected as DR or BDR regardless of failures on the network.Answer
  4. D
    Core-GW3 will preempt existing roles and trigger a complete re-election for both DR and BDR roles.

Answer

Core-GW3 will remain in the DROTHER state and will not be elected as DR or BDR regardless of failures on the network.
In OSPFv2 broadcast networks, the DR and BDR elections are determined first by the highest interface priority (default is 1, range is 0 to 255). A router interface configured with an OSPF priority of 0 is explicitly rendered ineligible to become either a DR or a BDR. Even if the current DR fails and even if Core-GW3 possesses the highest Router ID on the subnet (10.10.10.1010.10.10.10), its priority of 0 forces it to remain in the DROTHER state permanently.

Step-by-Step Solution

1
Examine the output of the command 'show ip ospf interface GigabitEthernet0/0'.
Identify that Core-GW3 has an OSPF interface priority of 0 ('Priority 0') and a Router ID of 10.10.10.10.
OSPF DR/BDR elections evaluate interface priority values before looking at Router IDs.
2
Apply the OSPF DR/BDR election priority rules.
Determine that an interface priority value of 0 explicitly disqualifies a router from participating in DR/BDR elections.
Setting 'ip ospf priority 0' configures the router to remain permanently as a DROTHER on broadcast and non-broadcast multiaccess networks.
3
Evaluate the network event (failure of Router-A).
Router-B (the existing BDR) will be promoted to DR. A new BDR will be elected from eligible DROTHER routers with priority > 0. Core-GW3 remains DROTHER.
Core-GW3 cannot participate in the election for the replacement BDR despite having a high Router ID.

Key Concept

OSPF DR/BDR Election Ineligibility with Priority 0
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Topics

OSPFv2 Network Types and DR/BDR Selection

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