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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

Router R1 and Router R2 are directly connected via their GigabitEthernet0/0 interfaces across an Ethernet link. A network engineer attempts to establish an OSPFv2 neighbor relationship between them in Area 0.

The interfaces are configured with the following parameters:
- Router R1 (Gi0/0): IP address 172.16.10.1/24 (Subnet Mask: 255.255.255.0), OSPF Area 0, Hello timer 10s, Dead timer 40s
- Router R2 (Gi0/0): IP address 172.16.10.2/25 (Subnet Mask: 255.255.255.128), OSPF Area 0, Hello timer 10s, Dead timer 40s

Executing `show ip ospf neighbor` on both routers reveals no neighbor entries. Which statement correctly explains why R1 and R2 fail to establish an OSPFv2 neighbor adjacency?

  1. A
    The OSPF process IDs configured on R1 and R2 do not match.
  2. The subnet masks configured on the interconnecting interfaces do not match.Answer
  3. C
    The routers are using different subnet lengths, which prevents basic IP unicast connectivity between 172.16.10.1 and 172.16.10.2.
  4. D
    The Dead timers are mismatched because the Dead timer is automatically derived by multiplying the Hello timer by the prefix length.

Answer

The correct explanation is that the subnet masks configured on the interconnecting interfaces do not match.
For OSPFv2 neighbor adjacencies to form over broadcast multiaccess or point-to-point networks, specific fields within the OSPF Hello packet must match across neighboring interfaces. One critical requirement is that the subnet mask of the primary IP address configured on the interface must be identical on both ends. Since R1 is configured with a /24 subnet mask and R2 is configured with a /25 subnet mask, the routers drop received Hello packets and fail to establish an OSPF neighbor relationship.

Step-by-Step Solution

1
Analyze OSPF Hello packet fields required for neighbor adjacency formation on broadcast/multiaccess interfaces.
OSPF Hello packets require matching parameters: Area ID, Subnet Mask, Hello and Dead timers, Authentication password/type, and Stub area flag.
If any of these mandatory fields mismatch, the routers drop received Hello packets and will not transition to the INIT or 2-WAY neighbor state.
2
Compare the interface configurations between R1 and R2.
R1 uses a /24 subnet mask (255.255.255.0) while R2 uses a /25 subnet mask (255.255.255.128).
Because the subnet masks differ, OSPF rejects the Hello packet exchange.
3
Verify secondary configuration items such as process IDs.
Process IDs are local to the router instance and do not affect neighbor adjacency formation.
Process IDs do not appear in Hello packets as adjacency requirements.

Key Concept

OSPFv2 Hello Packet Parameter Matching Requirements
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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