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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R1 and R2, are directly connected via their GigabitEthernet0/0 interfaces on the 192.168.12.0/24 network segment. OSPFv2 is initialized on both routers. R1 has its GigabitEthernet0/0 interface placed into OSPF Area 0, while R2 has its GigabitEthernet0/0 interface placed into OSPF Area 10. All other OSPF interface configurations use default settings. What will be the observed result when an administrator executes the command `show ip ospf neighbor` on R1?

  1. No neighbor entry for R2 will be displayed in the neighbor table.Answer
  2. B
    R2 will be permanently listed in the INIT state.
  3. C
    R2 will be stuck in the EXSTART state during Database Description packet negotiation.
  4. D
    R2 will reach the 2-WAY state but will fail to progress to FULL state.

Answer

No neighbor entry for R2 will be displayed in the neighbor table because Hello packets containing mismatched OSPF Area IDs are dropped during initial packet processing.
For OSPF neighbors to form an adjacency, specific Hello packet fields must match between connected interfaces: Area ID, Subnet Mask, Hello and Dead intervals, and Authentication. When R1 receives Hello packets from R2 with Area ID 10 while R1 is configured in Area 0, R1 discards the packets. Because valid Hello packets are never processed, R1 does not initialize a neighbor data structure for R2, leaving the OSPF neighbor table completely empty.

Step-by-Step Solution

1
Analyze OSPF Hello packet validation criteria
Identified that Area ID, Hello/Dead intervals, Subnet Mask (on broadcast networks), and Authentication parameters must match exactly between connected interfaces.
OSPF requires neighbor interfaces to share identical primary link configuration parameters before forming an adjacency.
2
Evaluate the effect of an Area ID mismatch
R1 (Area 0) receives Hello packets from R2 containing Area 10 and drops them immediately.
When a received OSPF Hello header contains an Area ID differing from the receiving interface's configured Area ID, the packet is invalidated.
3
Determine the resulting neighbor table state on R1
R1 does not record R2 in any OSPF state (Down/INIT/2-WAY/FULL).
Without processing incoming Hello packets, R1 never discovers R2, resulting in no entry in `show ip ospf neighbor`.

Key Concept

OSPFv2 Hello Packet Parameter Matching for Neighbor Adjacency
Estimated Time:1m 15s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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