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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Three Cisco routers (R1, R2, and R3) are connected to the same Layer 2 Ethernet switch on the 10.1.1.0/24 subnet and configured in OSPFv2 Area 0. Router R1 completes OSPF initialization first and is successfully elected as the Designated Router (DR). To optimize resource utilization, the network administrator explicitly configures the interface command `ip ospf priority 0` on GigabitEthernet0/0 on both R2 and R3.

After all OSPF processes converge, what neighbor state will be displayed when issuing the `show ip ospf neighbor` command on router R2 specifically for the neighbor entry corresponding to router R3?

  1. A
    FULL/DROTHER
  2. 2WAY/DROTHERAnswer
  3. C
    EXSTART/DR
  4. D
    INIT/DROTHER

Answer

The neighbor state between R2 and R3 will be 2WAY/DROTHER.
On a broadcast multiaccess network (such as Ethernet), routers only establish FULL adjacencies (exchanging link-state databases) with the Designated Router (DR) and Backup Designated Router (BDR). Routers configured with an OSPF priority of 0 cannot become DR or BDR (they remain DROTHERs). When two DROTHER routers discover each other via Hello packets, they verify bi-directional communication and remain stably in the 2WAY/DROTHER state.

Step-by-Step Solution

1
Analyze OSPF interface priority settings for R2 and R3.
Setting 'ip ospf priority 0' disables a router from participating in DR/BDR election, forcing both R2 and R3 into DROTHER status on the broadcast network.
Routers with an OSPF priority of 0 can never become a DR or BDR.
2
Evaluate OSPF adjacency requirements on broadcast multiaccess networks.
Full adjacencies are formed only between DROTHER routers and the DR/BDR. Routers that are both DROTHERs form relationships that stop at 2-WAY state.
This behavior minimizes flooding overhead and database synchronization traffic on multiaccess media.
3
Determine the output of the show command on R2 for R3.
The neighbor relationship successfully reaches bi-directional communication, yielding a state of 2WAY/DROTHER.
Both routers hear each other's Hellos (2-WAY), but neither router is a DR or BDR, so no LSA exchange takes place between them.

Key Concept

OSPF Adjacency Formation on Multiaccess Networks and Priority 0 Behavior
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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