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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

An administrator is troubleshooting an OSPFv2 neighbor relationship between two directly connected routers, R1 and R2. Output from the command `show ip ospf neighbor` on R1 displays the neighbor state for R2 as `INIT/ -`. Which condition is the most likely cause of this neighbor state?

  1. R1 is receiving OSPF Hello packets from R2, but R2 is not receiving OSPF Hello packets from R1.Answer
  2. B
    The maximum transmission unit (MTU) settings on the interconnecting interfaces of R1 and R2 do not match.
  3. C
    The OSPF process IDs configured on R1 and R2 do not match.
  4. D
    The OSPF priority on R1's interface has been configured with a value of 0.

Answer

R1 is receiving OSPF Hello packets from R2, but R2 is not receiving OSPF Hello packets from R1.
The INIT state indicates that a router has received an OSPF Hello packet from a neighbor, but bidirectional communication has not yet been verified because the receiving router's own Router ID is not listed in the neighbor's Hello packet. This typically occurs due to unidirectional link failures or filtering (such as an ACL blocking traffic in one direction).

Step-by-Step Solution

1
Analyze OSPF neighbor state definitions
Identify what the INIT state signifies in the OSPF neighbor state machine.
The INIT state means a valid OSPF Hello packet has been received from the neighbor, but two-way communication has not yet been established because the receiving router's own Router ID is missing from the neighbor's Hello Active Neighbor field.
2
Evaluate the cause of unidirectional Hello reception
Determine that R1 receives Hellos from R2, but R2 does not receive Hellos from R1.
If an access control list (ACL) or physical fault blocks packets in only one direction (from R1 to R2), R2 never sees R1's Hello, so R2 never includes R1's Router ID in its own sent Hellos. R1 continues receiving R2's Hellos and stays in the INIT state.
3
Differentiate from other failure states
Rule out MTU mismatch (ExStart/Exchange state), OSPF process ID mismatch (no impact), and priority 0 (prevents DR/BDR election only).
Understanding distinct neighbor states ensures accurate diagnosis of OSPF adjacency issues.

Key Concept

OSPFv2 Neighbor States and Unidirectional Communication
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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