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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

Two routers are connected across an Ethernet link and configured for OSPFv2 routing. Which TWO parameters exchanged in the OSPF Hello packet must match identically between the two routers for an OSPFv2 neighbor adjacency to form successfully?

  1. Area ID configured on the connected interfacesAnswer
  2. Hello and Dead timer intervalsAnswer
  3. C
    OSPF process ID numbers configured on each router
  4. D
    OSPF Router ID assigned to each router

Answer

The OSPF Area ID and the Hello and Dead timer intervals must match identically between neighboring routers.
For two OSPFv2 routers to establish a neighbor adjacency, specific parameters inside their Hello packets must be identical. These include the Area ID configured on the interface, the Hello and Dead timer intervals, the IP subnet mask (on broadcast/multiaccess links), and any stub/area option flags.

Step-by-Step Solution

1
Analyze OSPF Hello packet fields required for neighbor establishment
Identified mandatory match criteria: Area ID, Hello/Dead timers, Subnet Mask (on multiaccess networks), Stub area flag, and Authentication.
OSPF neighbor discovery requires agreement on basic link parameters before transitioning beyond the Init state.
2
Evaluate local vs global OSPF configuration settings
Process ID is purely local to the router process and is omitted from Hello packet negotiation.
Different routers in the same network can use different OSPF process IDs (e.g., Process 1 on R1 and Process 10 on R2).
3
Evaluate Router ID uniqueness requirements
Router IDs must be unique across the OSPF domain.
Duplicate Router IDs cause neighbor adjacencies to flap or fail to form.

Key Concept

OSPFv2 Hello Packet Mandatory Match Parameters
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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