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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R1 and R2, are directly connected via their GigabitEthernet 0/0 interfaces in OSPFv2 Area 0. An administrator configures `ip ospf hello-interval 5` on R1's GigabitEthernet 0/0 interface, while R2's interface retains the default OSPF Hello interval of 10 seconds. What is the impact of this configuration on the OSPF neighbor relationship between R1 and R2?

  1. The routers fail to establish an OSPF neighbor adjacency and the neighbor state remains down.Answer
  2. B
    The routers establish a neighbor relationship, but the adjacency is permanently stuck in the 2-WAY state.
  3. C
    The routers successfully form a FULL adjacency after automatically negotiating the Hello interval down to 5 seconds.
  4. D
    The routers form a FULL adjacency, but OSPF route recalculations occur twice as frequently on R1 than on R2.

Answer

The routers fail to establish an OSPF neighbor adjacency and the neighbor state remains down.
For two routers to establish an OSPFv2 neighbor relationship, essential parameters in their OSPF Hello packets must match exactly. These parameters include the Area ID, Hello interval, Dead interval, Subnet mask (on broadcast networks), and Authentication settings. Because the Hello interval on R1 is set to 5 seconds while R2 is using 10 seconds, both routers reject incoming Hello packets from one another, keeping the neighbor state in the Down state.

Step-by-Step Solution

1
Analyze OSPF Hello packet field validation requirements for neighbor adjacency formation.
Identified that specific fields in received Hello packets must match local interface configurations, including Area ID, Subnet Mask, Hello/Dead intervals, Authentication, and Stub flags.
OSPF routers verify these parameters before accepting a neighbor and placing it into the OSPF neighbor table.
2
Compare the configured Hello interval on R1 (5 seconds) with the default Hello interval on R2 (10 seconds).
A mismatch in Hello intervals exists across the link.
R1 expects Hello packets with a 5-second interval, while R2 sends Hello packets with a 10-second interval (and vice versa for Dead intervals).
3
Determine the resulting OSPF neighbor state.
Both routers drop incoming Hello packets from each other due to parameter mismatch, preventing neighbor discovery and adjacency formation.
Without matching Hello/Dead intervals, OSPF neighbor relationship setup cannot proceed past the Down state.

Key Concept

OSPFv2 Hello Packet Parameter Verification and Neighbor Adjacency Requirements
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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