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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, R1 and R2. Execution of the `show ip ospf neighbor` command on R1 reveals that the adjacency state with R2 remains persistently stuck in the EXSTART state. Which configuration mismatch is the primary cause of this behavior?

  1. The maximum transmission unit (MTU) size differs between the interconnecting interfaces on R1 and R2.Answer
  2. B
    The OSPF Area ID configured on R1's interface does not match the Area ID configured on R2's interface.
  3. C
    The Hello and Dead interval timers are configured with different values on R1 and R2.
  4. D
    R1 and R2 are configured with conflicting OSPF process IDs in their router configuration mode.

Answer

The maximum transmission unit (MTU) size differs between the interconnecting interfaces on R1 and R2.
When OSPF routers establish adjacency, they exchange Database Description (DBD) packets during the EXSTART and EXCHANGE states. By default, Cisco routers include the interface MTU in DBD packets. If the MTU values on the connecting interfaces do not match, the router with the larger MTU sends DBD packets that the other router drops, or the router receiving a higher MTU value rejects the DBD packet. Consequently, the neighbor relationship gets stuck in the EXSTART (or EXCHANGE) state.

Step-by-Step Solution

1
Identify mandatory OSPF Hello packet matching parameters versus post-Hello exchange parameters.
Hello packet parameters (Area ID, Hello/Dead timers, Subnet Mask, Authentication) must match for routers to establish a neighbor relationship.
If Hello parameters mismatch, routers ignore Hello packets and never transition past DOWN state.
2
Analyze OSPF neighbor state progression to EXSTART.
Transitioning to EXSTART indicates that Hello packets were accepted and the routers passed through INIT and 2-WAY states.
In EXSTART, routers negotiate Master/Slave roles and initial Sequence Numbers using Database Description (DBD) packets.
3
Determine the cause of persistent EXSTART/EXCHANGE state hangs.
An interface MTU mismatch causes DBD packets exceeding the smaller MTU to be dropped or ignored, leaving the neighbor stuck in EXSTART.
Cisco IOS checks the Interface MTU field in incoming DBD packets by default; if there is a mismatch, the exchange cannot complete.

Key Concept

OSPFv2 Adjacency Requirements and MTU Verification in EXSTART State
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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