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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

An administrator is troubleshooting an OSPFv2 adjacency issue between two directly connected Cisco routers, R1 and R2. Executing `show ip ospf neighbor` on R1 displays the neighbor state for R2 as stuck in `INIT/ -`. Which network condition or configuration error is the primary cause of this specific neighbor state?

  1. An Access Control List (ACL) on R2 is filtering incoming OSPF Hello packets sent by R1, creating unidirectional communication.Answer
  2. B
    The maximum transmission unit (MTU) size is mismatched on the connecting interfaces of R1 and R2.
  3. C
    The OSPF Hello and Dead interval timers are set to non-matching values on R1 and R2.
  4. D
    The connecting interfaces on R1 and R2 are assigned to different OSPF Area IDs.

Answer

An Access Control List (ACL) on R2 is filtering incoming OSPF Hello packets sent by R1, creating unidirectional communication.
The OSPF INIT state means a router has received a Hello packet from a neighbor, but the receiving router's own Router ID was not included in the neighbor's list of seen routers. This condition indicates one-way (unidirectional) communication. If an Access Control List on the remote router blocks incoming OSPF multicast or unicast traffic from the local router, the remote router will never record the local router's ID, causing the local router to remain stuck in the INIT state.

Step-by-Step Solution

1
Analyze the reported OSPF neighbor state
The neighbor state is `INIT/ -`.
The INIT state signifies that R1 has successfully received OSPF Hello packets from R2, but R1's own Router ID was not listed in R2's Hello packet active neighbor list.
2
Evaluate the cause of unidirectional Hello reception
R2 is sending Hellos to R1 (which R1 receives), but R2 is not receiving Hellos from R1.
If an ACL on R2 drops R1's incoming Hellos, R2 never processes R1's Hello, so R2 does not add R1's Router ID to its outgoing Hello packets.
3
Distinguish INIT state causes from DOWN and EXSTART state causes
Parameter mismatches (Area ID, Hello/Dead timers, Authentication) cause packets to be dropped before reaching INIT (remaining DOWN). MTU mismatches cause neighbors to freeze in EXSTART/EXCHANGE.
Only unidirectional traffic flow (such as an ACL blocking traffic in one direction) allows Hello reception without mutual Router ID acknowledgment.

Key Concept

OSPFv2 Neighbor States and Unidirectional Communication Troubleshooting
Estimated Time:1m 30s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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