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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network administrator executes the command `show ip ospf interface gigabitethernet 0/0` on two interconnecting routers to troubleshoot an OSPF neighbor issue.

Output from Router R1:
text
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.12.1/24, Area 0
Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Output from Router R2:
text
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.12.2/24, Area 1
Process ID 20, Router ID 2.2.2.2, Network Type BROADCAST
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Which condition is preventing R1 and R2 from establishing an OSPF neighbor adjacency?

  1. The OSPF Area IDs configured on the connected interfaces do not match.Answer
  2. B
    The OSPF Process IDs configured on R1 and R2 are different.
  3. C
    The routers are configured with different OSPF Router IDs.
  4. D
    The Dead timer interval is four times the length of the Hello timer interval.

Answer

The OSPF Area IDs configured on the connected interfaces do not match.
For two OSPF routers to establish a neighbor adjacency across a direct link, several key parameters in their exchanged Hello packets must match. One mandatory matching parameter is the Area ID. Because Router R1 is configured in Area 0 and Router R2 is configured in Area 1 on their interconnecting interfaces, the routers will discard each other's Hello packets and remain in the DOWN state.

Step-by-Step Solution

1
Inspect the CLI output for interface parameters sent in OSPF Hello packets.
Router R1 has interface GigabitEthernet0/0 assigned to Area 0, while Router R2 has interface GigabitEthernet0/0 assigned to Area 1.
For OSPF neighbors to form an adjacency over a common link, the Area ID field in their exchanged Hello packets must match exactly.
2
Verify other OSPF Hello packet parameters for compatibility.
Both routers share identical IP subnets (10.1.12.0/24) and matching timers (Hello 10, Dead 40). Both have unique Router IDs (1.1.1.1 and 2.2.2.2).
Process IDs (10 vs 20) are locally significant and do not affect neighbor relationships across a link.

Key Concept

OSPF Neighbor Adjacency Requirements
Estimated Time:1m 15s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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