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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers are directly connected via their GigabitEthernet 0/0 interfaces in OSPFv2 Area 0. Router R1 maintains the default interface timers. An administrator executes the command `ip ospf hello-interval 15` on interface GigabitEthernet 0/0 of Router R2, but makes no other configuration changes on either router. What is the impact of this configuration on the OSPF neighbor relationship?

  1. The routers fail to form an OSPF neighbor relationship because OSPF Hello and Dead intervals must match between directly connected neighbors.Answer
  2. B
    The routers successfully form a FULL neighbor relationship because Dead intervals automatically recalculate to four times the configured Hello interval.
  3. C
    The routers establish an adjacency but remain permanently stuck in the 2-WAY state.
  4. D
    The routers establish an adjacency, but routing update exchanges occur every 15 seconds instead of every 10 seconds.

Answer

The routers fail to form an OSPF neighbor relationship because OSPF Hello and Dead intervals must match between directly connected neighbors.
For two routers to form an OSPFv2 neighbor relationship on a link, specific parameters in their Hello packets must match. These include the Area ID, Subnet Mask, Hello Interval, Dead Interval, Authentication settings, and Stub flag. Modifying the Hello interval on Router R2 to 15 seconds causes a mismatch with Router R1's default 10-second Hello interval, leading both routers to reject incoming Hello packets.

Step-by-Step Solution

1
Identify the mandatory parameters required for OSPFv2 neighbor adjacency.
OSPF Hello packets contain field values that must match between neighbors: Area ID, Subnet Mask, Hello Interval, Dead Interval, Authentication Password/Digest, and Stub Area Flag.
If any mandatory parameter mismatches, the received Hello packet is dropped and neighbor discovery fails.
2
Analyze the effect of modifying the Hello interval on Router R2.
Router R2 sends Hello packets containing a Hello interval of 15 seconds and a Dead interval of 60 seconds (default 4×Hello4 \times \text{Hello}). Router R1 sends Hello packets with a Hello interval of 10 seconds and a Dead interval of 40 seconds.
Changing the Hello interval on one router creates a mismatch with the default Hello interval on the neighboring router.
3
Determine the resulting neighbor state.
Because the Hello and Dead intervals in the incoming Hello packets do not match the local interface settings, both routers drop each other's Hello packets, preventing an adjacency from forming.
Directly connected OSPF neighbors must have identical Hello and Dead timer values on the connecting interface segment.

Key Concept

OSPFv2 Hello and Dead Timer Matching Requirements
Estimated Time:1m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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