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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 and configured to run OSPFv2. During verification, a network engineer notices that the routers fail to form an OSPF neighbor relationship. Which TWO parameters configured on the interfaces or within the OSPF process must match identically in the OSPF Hello packets for the neighbor adjacency to successfully form? (Select TWO.)

  1. Area IDAnswer
  2. Hello and Dead timer intervalsAnswer
  3. C
    OSPF Process ID
  4. D
    OSPF Router ID

Answer

The parameters that must match identically for OSPFv2 neighbor adjacency formation are Area ID and Hello and Dead timer intervals.
To successfully establish an OSPFv2 neighbor adjacency, directly connected routers must agree on several Hello packet fields, including identical Area IDs, matching Hello and Dead timers, matching IP subnet/mask, matching authentication settings, and matching stub area flags. Area ID and Hello/Dead timer intervals are mandatory matching parameters.

Step-by-Step Solution

1
Analyze OSPF Hello packet contents required for neighbor validation.
Identify parameters that are verified upon receipt of a Hello packet on an OSPF-enabled interface.
For two OSPF routers to establish a neighbor relationship, key fields in the Hello packet header must match.
2
Evaluate the Area ID requirement.
Confirm that both connected interfaces must share the exact same Area ID.
If the Area ID field in the incoming Hello packet does not match the receiving interface's configured area, the packet is dropped.
3
Evaluate the Hello and Dead timer requirements.
Confirm that Hello intervals and Dead intervals must match exactly.
Mismatched timers prevent the routers from agreeing on keepalive expectations, causing adjacency initialization to fail.
4
Evaluate local vs global parameters (Process ID and Router ID).
Determine that Process ID is local only and Router ID must be unique.
Process ID is assigned locally per device, while Router IDs must never match to avoid duplicate router ID conflicts in the OSPF topology.

Key Concept

OSPFv2 Neighbor Adjacency Requirements
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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