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Zorluk: ZorOSPFv2 Neighbor Adjacencies and Router ID

A network administrator is troubleshooting an OSPFv2 connection between two directly connected routers, R1 and R2, over a point-to-point Ethernet link. Executing `show ip ospf neighbor` on R1 reveals that the neighbor state for R2 is perpetually stuck in the `EXSTART/ -` state.

Which misconfiguration is the most likely root cause of this neighbor adjacency state?

  1. The interface MTU setting on R1 does not match the interface MTU setting on R2.Cevap
  2. B
    The OSPF Hello and Dead interval timers are mismatched on the connecting interfaces.
  3. C
    The routers are configured with conflicting OSPF Area IDs on their connecting interfaces.
  4. D
    Both routers have been configured with identical OSPF Router IDs.

Cevap

The interface MTU setting on R1 does not match the interface MTU setting on R2.
In OSPFv2, routers exchange Database Description (DBD) packets during the EXSTART and EXCHANGE states to negotiate Master/Slave roles and initial sequence numbers. The DBD packet header explicitly contains the originating interface's MTU setting. If the MTU values on the interconnected interfaces do not match, the router with the smaller MTU will drop DBD packets that exceed its size or reject incoming DBDs carrying a higher MTU value. As a result, the neighbor negotiation stalls and remains stuck in the EXSTART (or EXCHANGE) state.

Adım Adım Çözüm

1
Analyze OSPF Neighbor States and Hello exchange requirements
Parameters like Area ID, Hello/Dead timers, Subnet Mask, and Authentication must match in Hello packets to form 2-Way neighbor state.
If Hello packet parameters mismatch, the routers cannot form a bidirectional 2-Way relationship.
2
Analyze the EXSTART/EXCHANGE state behavior
In EXSTART, routers elect a Master/Slave relationship and exchange initial Database Description (DBD) packets containing the interface MTU value.
OSPF includes its interface MTU in the DBD packet header. If receiving router MTU is lower than sender MTU (or MTU mismatch enforcement is active), DBD packets are ignored or dropped.
3
Determine the root cause of neighbor stuck in EXSTART state
Mismatched interface MTU values cause DBD retransmissions and prevent the routers from completing ExStart/Exchange into Loading and Full states.
The correct solution identifies interface MTU mismatch as the primary cause of an OSPF neighbor relationship becoming stuck in EXSTART.

Anahtar Kavram

OSPFv2 Neighbor States and MTU Mismatch Behavior
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OSPFv2 Neighbor Adjacencies and Router ID

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