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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

A network administrator views the OSPFv2 status of interface GigabitEthernet0/0 on router R1:

text
R1# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
State DROTHER, Priority 0
Designated Router (ID) 3.3.3.3, Interface address 10.10.10.3
Backup Designated router (ID) 2.2.2.2, Interface address 10.10.10.2

If the current Designated Router (Router ID 3.3.3.3) unexpectedly reboots and fails on this multiaccess segment, which behavior will R1 exhibit during the subsequent election process?

  1. A
    R1 will compete for the Backup Designated Router (BDR) role because its Router ID is evaluated after the current BDR promotes to DR.
  2. R1 will remain in the DROTHER state and will not participate in the election for either the DR or BDR role.Answer
  3. C
    R1 will automatically become the new DR because it has an active OSPF interface configured on the subnet.
  4. D
    R1 will temporarily assume the BDR role until a router with a non-zero priority joins the broadcast domain.

Answer

R1 will remain in the DROTHER state and will not participate in the election for either the DR or BDR role.
In OSPFv2 multiaccess networks, setting `ip ospf priority 0` on an interface makes that interface completely ineligible to participate in Designated Router (DR) or Backup Designated Router (BDR) elections. When the primary DR fails, the existing BDR (Router ID 2.2.2.2) promotes to DR, and a new BDR is elected only among other eligible routers with priority values of 1 to 255. Therefore, the router displaying priority 0 will remain in the DROTHER state throughout the election process.

Step-by-Step Solution

1
Analyze the CLI output for interface priority.
Interface GigabitEthernet0/0 has 'Priority 0'.
In OSPFv2, the interface priority parameter dictates DR/BDR election eligibility.
2
Apply OSPF DR/BDR election rules for priority 0.
A router interface with an OSPF priority value of 0 is ineligible to become either DR or BDR.
Priority 0 is a explicitly defined mechanism to prevent a router interface from ever becoming a Designated Router or Backup Designated Router on broadcast or non-broadcast multiaccess networks.
3
Determine the state transition when DR fails.
The current BDR (Router ID 2.2.2.2) promotes to DR, and a new BDR election occurs among eligible routers (priority > 0). R1 remains DROTHER.
Because R1 is ineligible, it cannot be elected as BDR or DR during the promotion or re-election phase.

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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