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Difficulty: EasyOSPFv2 Neighbor Adjacencies and Router ID

Two directly connected Cisco routers, R1 and R2, are configured for OSPFv2 on their GigabitEthernet0/0 interfaces. Router R1 has its interface configured in OSPF Area 0, while Router R2 has its connecting interface configured in OSPF Area 1. All physical interfaces and IP subnet parameters are operating correctly.

What is the expected OSPF neighbor outcome between R1 and R2?

  1. The routers will fail to establish an OSPF neighbor relationship because Area IDs in Hello packets must match.Answer
  2. B
    The routers will form a 2-Way neighbor state, but they will fail to exchange routing tables and reach Full state.
  3. C
    The routers will successfully form a Full adjacency as long as both routers use the same OSPF Process ID.
  4. D
    The routers will form a Full neighbor adjacency only after a Designated Router (DR) is elected on the link.

Answer

The routers will fail to establish an OSPF neighbor relationship because Area IDs in Hello packets must match.
For two routers to form an OSPFv2 neighbor relationship, specific fields in their Hello packets must match exactly on the interconnecting link. These required matching parameters include the Area ID, Subnet Mask, Hello and Dead intervals, Primary IP Subnet, Authentication credentials, and Stub Area flag. Because R1 is configured in Area 0 and R2 is configured in Area 1, their Hello packets will be dropped by the receiving interfaces, and no neighbor relationship will be established.

Step-by-Step Solution

1
Analyze OSPF Hello packet matching requirements for neighbor adjacency.
Identify that the Area ID field is a mandatory matching parameter in OSPFv2 Hello packets.
For two routers to establish any OSPF neighbor state (starting from Init), primary parameters in the Hello packet must match exactly.
2
Compare the interface configuration settings of R1 and R2.
R1 is in Area 0 and R2 is in Area 1.
The Area IDs on the interconnecting link are mismatched.
3
Determine the impact of the Area ID mismatch on neighbor discovery.
R1 drops Hello packets received from R2 (and vice versa), preventing neighbor formation completely.
When a router receives an OSPF Hello packet with a mismatched Area ID, it drops the packet without adding the sending router to its neighbor table.

Key Concept

OSPFv2 Neighbor Adjacency Requirements
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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