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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Refer to the following command output from Router R1 on a multiaccess Ethernet network:

text
R1# show ip ospf interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 192.168.10.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 3.3.3.3, Interface address 192.168.10.3
Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.10.2

The administrator attempts to force R1 to become the Designated Router (DR) by assigning it the highest Router ID on the segment (10.10.10.10) and resetting the OSPF process. However, R1 remains in the DROTHER state. Why did R1 fail to participate in the DR/BDR election?

  1. The OSPF interface priority on R1 is set to 0, which explicitly excludes the router from participating in DR/BDR elections regardless of its Router ID.Answer
  2. B
    The active DR (3.3.3.3) has non-preemptive election rights, so changing R1's Router ID cannot trigger a re-election until the DR fails.
  3. C
    The OSPF network type is BROADCAST, which requires manual neighbor configuration to elect a new DR.
  4. D
    R1's IP address (192.168.10.1) is lower than the current DR's IP address (192.168.10.3), preventing it from taking over the DR role.

Answer

R1 cannot become the Designated Router because its OSPF interface priority is configured as 0, which makes the interface ineligible to participate in DR/BDR elections.
In OSPFv2 multiaccess networks (such as Broadcast), DR/BDR elections evaluate interface priority first. A router with an interface priority of 0 is strictly excluded from becoming a DR or BDR and will always remain a DROTHER, regardless of how high its Router ID or interface IP address is.

Step-by-Step Solution

1
Examine the output of 'show ip ospf interface GigabitEthernet0/0'.
Identify that R1 has an OSPF Priority of 0 and a State of DROTHER.
Interface priority determines DR/BDR election eligibility.
2
Evaluate the effect of OSPF interface priority 0.
Recognize that a priority value of 0 explicitly prevents a router from ever becoming a DR or BDR.
OSPF rules dictate that any interface with priority 0 remains permanently in the DROTHER state regardless of Router ID or interface IP address.

Key Concept

OSPF Priority 0 Exclusion in DR/BDR Selection
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Topics

OSPFv2 Network Types and DR/BDR Selection

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