Examkin

Question

Difficulty: EasyOSPFv2 Neighbor Adjacencies and Router ID

During network troubleshooting, an administrator discovers that Router A is receiving OSPFv2 Hello packets from directly connected Router B. However, Router B is dropping all incoming Hello packets from Router A because of an inbound interface access list. In which OSPF neighbor state will Router A remain with respect to Router B?

  1. InitAnswer
  2. B
    2-Way
  3. C
    ExStart
  4. D
    Attempt

Answer

Router A will remain in the Init neighbor state.
When a router receives an OSPF Hello packet from a neighbor, it places that neighbor into the Init state. To transition to the 2-Way state, bidirectional communication must be verified—meaning the receiving router must see its own Router ID listed inside the neighbor's Hello packet. Because the access list blocks Router A's Hellos from reaching Router B, Router B never learns of Router A and never includes Router A's Router ID in its Hellos. Consequently, Router A remains stuck in the Init state.

Step-by-Step Solution

1
Analyze the flow of OSPF Hello packets between Router A and Router B.
Router A receives Hello packets sent by Router B, but Router B does not receive Hello packets from Router A due to the inbound access list.
Unidirectional communication is occurring.
2
Evaluate the requirement for transitioning out of the Init state.
To move from Init to 2-Way state, Router A must see its own Router ID listed in the Neighbor field of Router B's Hello packet.
This verifies bidirectional communication between the two neighbors.
3
Determine Router A's resulting neighbor state.
Because Router B never receives Router A's Hellos, Router B never includes Router A's Router ID in its Hello packets. Router A remains stuck in the Init state.
Receiving a Hello packet without seeing one's own Router ID keeps the neighbor in the Init state.

Key Concept

OSPF Neighbor State Machine (Init State & Bidirectional Communication)
Rate this question

Topics

OSPFv2 Neighbor Adjacencies and Router ID

More questions from this topic