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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

A network administrator is troubleshooting an OSPFv2 adjacency issue between two routers, R1 and R2, connected via their GigabitEthernet0/0/0 interfaces. The administrator notices that the OSPF neighbor state on R1 remains persistently stuck in the EXSTART/EXCHANGE state. Interface GigabitEthernet0/0/0 on R1 has an IP MTU of 1500 bytes, while GigabitEthernet0/0/0 on R2 has been configured with an IP MTU of 1450 bytes. If modifying the interface MTU on R2 is prohibited by organizational policy, which command configured on R1 will allow the OSPF neighbor adjacency to proceed successfully to the FULL state?

  1. Configure ip ospf mtu-ignore under interface GigabitEthernet0/0/0 on R1.Answer
  2. B
    Configure ip ospf mtu-ignore under the router ospf 1 configuration prompt on R1.
  3. C
    Configure ip ospf priority 0 under interface GigabitEthernet0/0/0 on R1.
  4. D
    Configure clear ip ospf process under execution mode on R1 to reset neighbor negotiation parameters.

Answer

Configure ip ospf mtu-ignore under interface GigabitEthernet0/0/0 on R1.
During the EXSTART state, OSPF routers exchange Database Descriptor (DBD) packets to agree on initial sequence numbers and master/slave roles. The DBD packet includes the interface MTU. If neighboring routers have mismatched MTU values, DBD packet processing fails and the neighbor relationship remains stuck in EXSTART/EXCHANGE. Configuring 'ip ospf mtu-ignore' in interface configuration mode forces the router to ignore MTU checks in received DBD packets, allowing the adjacency to transition to FULL.

Step-by-Step Solution

1
Identify the cause of the EXSTART/EXCHANGE neighbor state stagnation.
During the EXSTART and EXCHANGE states, OSPF neighbors elect a Master/Slave relationship and exchange Database Descriptor (DBD) packets containing interface MTU values.
If the receiving interface detects an MTU larger than its own or mismatched MTU parameters, the DBD packet processing fails and the neighbor relationship halts at EXSTART/EXCHANGE.
2
Determine the appropriate configuration workaround when interface MTU modification is restricted.
Cisco IOS provides the 'ip ospf mtu-ignore' interface configuration command.
This command disables the MTU consistency check on incoming OSPF DBD packets for that specific interface, permitting adjacency completion.

Key Concept

OSPFv2 MTU Mismatch Behavior and ip ospf mtu-ignore
Estimated Time:2m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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