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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R1 and R2, are directly connected via an Ethernet link and configured for OSPFv2 single-area routing. An administrator notices that the OSPF neighbor state between R1 and R2 remains continuously stuck in the EXSTART state. Which configuration mismatch is the most likely cause of this issue?

  1. An IP Maximum Transmission Unit (MTU) mismatch on the interconnecting interfacesAnswer
  2. B
    Mismatched OSPF Hello or Dead timer intervals on the interface
  3. C
    Different OSPF Area IDs assigned to the connecting interfaces
  4. D
    Setting the OSPF interface priority to 0 on both routers

Answer

An IP Maximum Transmission Unit (MTU) mismatch on the interconnecting interfaces causes OSPF neighbors to become stuck in the ExStart or Exchange state.
During the ExStart state of OSPFv2 neighbor formation, routers elect a Master and Slave to sequence Database Description (DBD) packets. Cisco routers validate the IP MTU header field in incoming DBD packets against their local interface MTU. If an MTU mismatch exists, the larger-MTU router drops the smaller-MTU router's DBD packets (or vice versa), keeping the neighbor process continuously stuck in ExStart/Exchange.

Step-by-Step Solution

1
Analyze OSPF neighbor establishment states
Identify that Hello parameter mismatches (Area ID, Hello/Dead timers, Authentication, Subnet Mask) prevent initial Hello acceptance and stop adjacencies in Down or Init states.
Basic parameters must match before Database Description (DBD) packet exchange begins.
2
Examine the ExStart/Exchange neighbor state requirements
In ExStart state, routers negotiate master/slave roles using DBD packets. By default, Cisco routers verify that the IP MTU in received DBD packets does not exceed their local interface MTU.
If MTU sizes differ between interfaces, DBD packet exchanges fail or are dropped, leaving the neighbor relationship stuck in ExStart or Exchange.

Key Concept

OSPF Neighbor States and MTU Mismatch Troubleshooting
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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