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Difficulty: EasyOSPFv2 Neighbor Adjacencies and Router ID

Two directly connected Cisco routers are configured with OSPFv2 on their connecting interfaces. Router R1 has its interface assigned to OSPF Area 0, while Router R2 has its connecting interface assigned to OSPF Area 1. Assuming all other OSPF parameters are left at default settings, what is the result of this configuration?

  1. The routers fail to form an OSPF neighbor adjacency because the Area IDs do not match.Answer
  2. B
    The routers progress to the 2-WAY state but remain in that state without exchanging link-state database information.
  3. C
    The routers form a FULL adjacency, but routes learned from Area 1 are filtered from the Area 0 routing table.
  4. D
    The routers become stuck in the ExStart state due to an inability to negotiate master/slave roles.

Answer

The routers fail to form an OSPF neighbor adjacency because the Area IDs do not match.
For two OSPFv2 routers to establish a neighbor adjacency, several interface parameters exchanged in Hello packets must match exactly. One mandatory parameter is the Area ID. Because one router's interface is configured in Area 0 and the other in Area 1, the routers reject each other's Hello packets and fail to form an adjacency.

Step-by-Step Solution

1
Identify the mandatory matching parameters required in OSPF Hello packets for neighbor formation.
Area ID, Subnet Mask, Hello/Dead Intervals, Authentication, and Option flags (such as Stub area flags) must match between neighbors on a link.
OSPF routers validate these fields upon receiving a Hello packet before accepting the sending router as a valid neighbor.
2
Evaluate the effect of placing connecting interfaces into different areas (Area 0 vs. Area 1).
Incoming OSPF Hello packets are dropped due to the Area ID mismatch.
Because the Area ID field in the Hello header does not match the local interface's configured area, neighbor discovery fails immediately.

Key Concept

OSPFv2 Neighbor Adjacency Requirements
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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