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Zorluk: ZorOSPFv2 Neighbor Adjacencies and Router ID

A network engineer is troubleshooting an OSPFv2 neighbor relationship between router R1 and router R2 connected across a shared Ethernet segment. Command output from `show ip ospf neighbor` on R1 displays R2 in the `INIT/ -` state. Which condition is the primary cause of the neighbor relationship being stuck in the INIT state?

  1. An access control list is blocking OSPF Hello packets sent from R1 to R2, resulting in unidirectional communicationCevap
  2. B
    An IP Maximum Transmission Unit (MTU) size mismatch exists between the connecting interfaces of R1 and R2
  3. C
    A mismatch exists between the OSPF Area ID configured on R1 and R2 for the connecting subnets
  4. D
    The OSPF Hello and Dead interval timers are configured with non-matching values on the two routers

Cevap

An access control list is blocking OSPF Hello packets sent from R1 to R2, resulting in unidirectional communication.
The INIT state indicates that R1 has successfully received OSPF Hello packets from R2, but R1 has not seen its own Router ID listed in R2's Hello packet. This scenario occurs when traffic flow is unidirectional—such as when an inbound or outbound Access Control List (ACL) blocks OSPF Hello packets transmitted from R1 to R2 while allowing Hellos from R2 to reach R1.

Adım Adım Çözüm

1
Analyze OSPF neighbor state machine progression for the INIT state
The INIT state means a router has received an OSPF Hello packet from a neighbor, but its own Router ID is NOT present in the list of seen neighbors contained within that Hello packet.
Transitioning beyond INIT to 2-Way requires bi-directional communication, where both routers acknowledge receiving each other's Hello packets.
2
Evaluate the cause of unidirectional Hello reception
If R1 receives R2's Hellos, R1 transitions R2 to the INIT state. If R2 never receives R1's Hellos (e.g., due to an ACL filtering packets from R1 to R2), R2 will not include R1's Router ID in its outgoing Hellos.
Without R1's Router ID in R2's Hello packet, R1 cannot transition the neighbor relationship to 2-Way, leaving it permanently stuck in INIT.
3
Distinguish INIT state causes from DOWN and EXSTART state causes
Mismatches in Area ID, Hello/Dead timers, subnet mask, or authentication cause Hello packets to be dropped on arrival, keeping the neighbor in DOWN state. MTU mismatches cause neighbors to get stuck in EXSTART/EXCHANGE state.
Identifying which parameters are checked during Hello validation pinpoints why other mismatches fail earlier (DOWN) or later (EXSTART) in the adjacency process.

Anahtar Kavram

OSPF Neighbor Adjacency States and Unidirectional Communication Troubleshooting
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Konular

OSPFv2 Neighbor Adjacencies and Router ID

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