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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R1 and R2, are directly connected via their GigabitEthernet0/0 interfaces in OSPFv2 Area 0. Interface GigabitEthernet0/0 on R1 is configured with a maximum transmission unit (MTU) of 1500 bytes, whereas GigabitEthernet0/0 on R2 is configured with an MTU of 1400 bytes. Neither router has the ip ospf mtu-ignore command configured.

In which specific OSPF neighbor state will the adjacency between R1 and R2 become stuck?

Answer: ExStart / EXSTART / ExStart state / EXSTART state

Answer

ExStart
When OSPF neighbor parameters in Hello packets match, routers establish two-way communication and progress to the ExStart state to begin Database Description (DBD) packet negotiation. Because interface MTU values are advertised inside DBD headers, an MTU mismatch (1500 bytes on R1 vs. 1400 bytes on R2) causes the router with the lower MTU to ignore incoming DBD packets. As a result, Master/Slave election and DBD sequence number synchronization fail, leaving the neighbor relationship stuck in the ExStart state.

Step-by-Step Solution

1
Analyze OSPF Hello packet processing and initial state transitions
Hello parameters (Area ID, Subnet Mask, Hello/Dead timers, Authentication) match between R1 and R2, allowing the routers to pass the Init state and transition into the 2-Way state.
MTU values are not contained or evaluated within OSPF Hello packets.
2
Evaluate Database Description (DBD) packet negotiation during ExStart
Upon transitioning to the ExStart state, routers exchange Database Description (DBD) packets to elect a Master/Slave relationship and negotiate sequence numbers. The configured interface MTU is explicitly included in the Interface MTU field of the DBD header.
OSPF requires matching interface MTUs to ensure Database Description packets carrying Link-State Advertisements can be exchanged without fragmentation issues.
3
Determine the impact of the MTU mismatch
R2 receives a DBD packet from R1 advertising an MTU of 1500 bytes, which exceeds R2's interface MTU of 1400 bytes. Consequently, R2 drops the packet, causing Master/Slave negotiation to stall endlessly in the ExStart state.
Without ip ospf mtu-ignore enabled, any mismatch in advertised MTU prevents completion of DBD packet exchange.

Key Concept

OSPF Neighbor States and MTU Mismatch Behavior
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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