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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network administrator configures OSPFv2 across four Cisco routers connected to a single Ethernet switch on a broadcast multiaccess segment. After allowing time for network convergence, the administrator issues the `show ip ospf neighbor` command on Router R3, which is neither the Designated Router (DR) nor the Backup Designated Router (BDR). The output shows that the neighbor relationship with Router R4 (also a DROther) is in the `2WAY/DROTHER` state. Which statement correctly describes this scenario?

  1. This is normal expected behavior because non-DR/BDR routers on a broadcast network form bi-directional communication but do not proceed to full database synchronization with each other.Answer
  2. B
    The routers are stuck due to an Interface Maximum Transmission Unit (MTU) size mismatch preventing the exchange of Database Description (DBD) packets.
  3. C
    Router R4 is failing to process incoming OSPF Hello packets from R3 due to a mismatched OSPF process ID configured on the local interface.
  4. D
    The neighbor relationship failed to reach the FULL state because the OSPF Hello and Dead interval timers are improperly configured on R4.

Answer

This is normal expected behavior because non-DR/BDR routers on a broadcast network form bi-directional communication but do not proceed to full database synchronization with each other.
On broadcast multiaccess networks (such as Ethernet), OSPF elects a Designated Router (DR) and Backup Designated Router (BDR). All other routers on the segment are designated as DROthers. To minimize routing overhead and redundant LSA exchanges, DROthers only establish FULL adjacencies with the DR and BDR. Between two DROther routers, bi-directional communication is established and maintained at the 2-WAY state, which is normal and expected behavior.

Step-by-Step Solution

1
Analyze the network topology and router roles
The network is a broadcast multiaccess Ethernet segment with four OSPFv2 routers. R3 and R4 are identified as DROther routers (neither DR nor BDR).
OSPF behaves differently on broadcast networks depending on whether the routers involved are DR/BDR or DROthers.
2
Evaluate the OSPF neighbor state machine logic for DROther to DROther communications
DROther routers achieve 2-WAY state with one another once bi-directional Hello exchange succeeds.
To prevent redundant LSA exchanges and conserve network resources on multiaccess media, full database synchronization (FULL state) only occurs between DROthers and the DR/BDR.
3
Determine if troubleshooting is required for the 2WAY/DROTHER status
No troubleshooting is needed; the neighbor state is operating normally as designed by OSPF standards.
Reaching 2-WAY state confirms correct matching of Area ID, subnet mask, authentication, MTU handling, and Hello/Dead timers.

Key Concept

OSPF DR/BDR neighbor adjacency states on broadcast networks
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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