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Zorluk: ZorOSPFv2 Neighbor Adjacencies and Router ID

A network administrator is troubleshooting an OSPFv2 neighbor relationship between two Cisco routers, Edge-R1 and Core-R1. Running the command 'show ip ospf neighbor' on Edge-R1 indicates that Core-R1 is persistently stuck in the INIT state. Verification confirms that both router interfaces are assigned to Area 0, have matching IP subnet masks, and share identical Hello and Dead timer intervals. Which underlying condition is the most likely cause of the neighbor relationship remaining stuck in the INIT state?

  1. An Access Control List (ACL) on Core-R1 is filtering incoming OSPF Hello packets sent by Edge-R1.Cevap
  2. B
    The Maximum Transmission Unit (MTU) configured on Edge-R1's interface does not match the MTU configured on Core-R1's interface.
  3. C
    Edge-R1 and Core-R1 are configured with different OSPF process IDs in their global routing configuration.
  4. D
    The OSPF priority on Core-R1's interface is set to 0, preventing it from participating in Designated Router elections.

Cevap

An Access Control List (ACL) on Core-R1 is filtering incoming OSPF Hello packets sent by Edge-R1.
The correct answer identifies that an inbound ACL on the remote router is filtering Hello packets in one direction. In OSPF, a router transitions a peer to the INIT state upon receiving a Hello packet. To progress to 2-WAY state, the router must see its own Router ID in the active neighbor list of the peer's incoming Hello packet. If an ACL blocks Hellos in one direction, the peer never receives the Hellos, never lists the local router's ID, and leaves the local router stuck in the INIT state.

Adım Adım Çözüm

1
Analyze OSPF neighbor state progression mechanics.
Transition to the 2-WAY state requires bidirectional communication, meaning a router must see its own Router ID listed in the Received Hello packet's neighbor list from its peer.
When Edge-R1 sees Core-R1's Hello packet, it places Core-R1 into its neighbor table in the INIT state.
2
Evaluate the impact of unidirectionally dropped Hello packets.
Edge-R1 is receiving Hellos from Core-R1, but Core-R1 is NOT receiving Hellos from Edge-R1 due to an inbound ACL filter on Core-R1.
Because Core-R1 never receives Edge-R1's Hellos, Core-R1 cannot add Edge-R1's Router ID to its outgoing Hello packet neighbor list. Thus, Edge-R1 stays stuck in INIT.
3
Differentiate INIT state causes from EXSTART state and configuration parameters.
MTU mismatches cause EXSTART/EXCHANGE stuck states, whereas mismatched process IDs or priority 0 do not prevent FULL neighbor adjacency.
Only unidirectional traffic flow (or filtering of Hellos in one direction) accounts for a persistent INIT state when basic parameters like Area ID and timers match.

Anahtar Kavram

OSPFv2 Neighbor States and Bidirectional Hello Communication Requirements
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OSPFv2 Neighbor Adjacencies and Router ID

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