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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R1 and R2, are connected via a dedicated serial link configured for OSPFv2 Area 0. A network engineer runs the command `show ip ospf neighbor` on R1 and notices that the neighbor relationship with R2 remains persistently stuck in the INIT state. Which condition is the most likely cause of this neighbor state?

  1. R2 is receiving R1's OSPF Hello packets, but R2 is rejecting them due to a mismatched Hello/Dead interval and omitting R1's Router ID from its own Hello packets.Answer
  2. B
    An Maximum Transmission Unit (MTU) mismatch exists between R1 and R2, preventing the exchange of Database Description (DBD) packets.
  3. C
    R1's interface OSPF priority is set to 0, which disables its ability to form neighbor adjacencies on point-to-point links.
  4. D
    An inbound Access Control List (ACL) on R1 is blocking incoming multicast traffic sent to destination address 224.0.0.5.

Answer

R2 is receiving R1's OSPF Hello packets, but R2 is rejecting them due to a mismatched Hello/Dead interval and omitting R1's Router ID from its own Hello packets.
The correct answer identifies that the INIT state occurs when a local router receives a Hello packet from a peer, but the peer does not list the local router's Router ID in its Hello neighbor field. This typically happens when the peer drops the local router's Hello packets due to mismatched Hello/Dead timers or Area IDs.

Step-by-Step Solution

1
Analyze what the OSPF INIT state signifies.
The INIT state means a router has received an OSPF Hello packet from a neighbor, but the receiving router's own Router ID is NOT listed in the neighbor field of that received Hello packet.
OSPF requires bidirectional communication. A router only moves past INIT to 2-WAY once it verifies that its peer acknowledges seeing it by listing its Router ID in outbound Hellos.
2
Determine why R2 would omit R1's Router ID from its Hello packets.
R2 is dropping R1's incoming Hello packets because of mismatched parameters (such as Hello/Dead intervals, Area ID, or authentication passwords).
Because R2 discards R1's incoming Hellos, R2 never processes R1 as a valid neighbor and does not include R1's Router ID in the neighbor list of its own transmitted Hello packets.
3
Evaluate the incorrect options against OSPF state mechanics.
MTU mismatches freeze adjacencies at ExStart/Exchange; priority 0 affects DR/BDR election eligibility; blocking inbound Hellos at R1 keeps the state DOWN.
Only an asymmetric drop of Hello packets by the peer router explains why R1 hears R2 (moving R1 to INIT) while R2 fails to acknowledge R1.

Key Concept

OSPFv2 Neighbor States and Hello Parameter Verification
Estimated Time:2m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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