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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network administrator is troubleshooting an OSPFv2 neighbor relationship between two directly connected Cisco routers. Command-line interface output indicates that the neighbor relationship is persistently stuck in the ExStart state. Which configuration mismatch is the primary cause of this specific neighbor state condition?

  1. Interface Maximum Transmission Unit (MTU) sizeAnswer
  2. B
    OSPF Hello and Dead timer intervals
  3. C
    OSPF Area Identifier configuration
  4. D
    IP subnet mask on the connecting link

Answer

Interface Maximum Transmission Unit (MTU) size
During OSPF adjacency negotiation, routers pass through Init and 2-Way before entering ExStart. In ExStart, master/slave roles are determined using Database Description (DBD) packets, which contain the interface MTU size. If the interface MTUs between neighbor interfaces do not match, the router receiving DBDs with a higher MTU drops them, causing the neighbor state to remain stuck in ExStart.

Step-by-Step Solution

1
Analyze OSPF Hello packet verification parameters.
Parameters verified in Hello packets include Area ID, Hello/Dead timers, Subnet Mask, and Authentication. A mismatch in any of these parameters causes Hello packets to be ignored, leaving neighbors in the Down state.
Interface MTU is not checked during the initial Hello packet exchange.
2
Trace adjacency progression into the ExStart state.
Because initial Hello parameters match, the routers establish two-way communication (2-Way) and progress to ExStart to begin Database Description (DBD) packet negotiation.
ExStart is the state where master/slave roles and initial Database Description sequence numbers are negotiated.
3
Identify the parameter evaluated during DBD packet exchange.
DBD packets carry the configured interface MTU value. If one router receives a DBD packet specifying an MTU larger than its own configured interface MTU, it drops the packet.
Dropping DBD packets prevents sequence number acknowledgment, trapping the neighbor relationship in the ExStart/Exchange state.

Key Concept

OSPF Neighbor States and MTU Mismatch Impact
Estimated Time:1m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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