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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Two Cisco routers, R1 and R2, are directly connected over their GigabitEthernet0/0 interfaces in OSPFv2 Area 0. Interface GigabitEthernet0/0 on R1 has an IP address of 172.16.12.1/24172.16.12.1/24 with a default IP MTU of 15001500 bytes. Interface GigabitEthernet0/0 on R2 has an IP address of 172.16.12.2/24172.16.12.2/24 and its IP MTU has been modified to 14001400 bytes. Both routers successfully exchange Hello packets and recognize each other as neighbors, but the output of `show ip ospf neighbor` on R1 shows R2 perpetually stuck in the `EXSTART` state.

If the network administrator cannot alter the IP MTU size on either router interface, which configuration command must be configured under interface GigabitEthernet0/0 to allow the OSPF neighbor relationship to reach the `FULL` state?

  1. Apply the `ip ospf mtu-ignore` command under the interface configuration mode on either or both routers.Answer
  2. B
    Apply the `ip ospf hello-interval 10` command under the interface configuration mode on R2.
  3. C
    Reconfigure R2 with a higher OSPF Router ID using the `router-id 3.3.3.3` command under `router ospf` mode.
  4. D
    Configure `ip ospf network point-to-point` under the interface configuration mode on both connected interfaces.

Answer

The neighbor relationship can reach the FULL state by configuring the `ip ospf mtu-ignore` command on the interface with the MTU mismatch.
The correct option applies `ip ospf mtu-ignore` under interface configuration mode. When OSPF routers transition from 2-WAY to EXSTART, they negotiate Master/Slave roles and exchange Database Description (DBD) packets. Each router includes its interface MTU in the DBD packet. If there is an MTU mismatch, the router receiving the larger MTU value drops or ignores the DBD packet, keeping the adjacency stuck in EXSTART (or EXCHANGE). Executing `ip ospf mtu-ignore` forces OSPF to ignore the MTU value in incoming DBD packets, allowing the exchange to proceed to the FULL state.

Step-by-Step Solution

1
Analyze the OSPF neighbor state
Neighbors reached the EXSTART state, confirming that Hello packet parameters (Area ID, Subnet Mask, Hello/Dead timers, Authentication) match.
Mismatched Hello parameters prevent neighbors from reaching 2-WAY or EXSTART entirely.
2
Identify the cause of the EXSTART state lockup
R1 has an IP MTU of 15001500 bytes while R2 has an IP MTU of 14001400 bytes.
During the EXSTART/EXCHANGE state, routers exchange Database Description (DBD) packets containing the interface MTU field. If a router receives a DBD packet specifying an MTU higher than its local interface MTU, it will reject the packet and remain stuck in EXSTART/EXCHANGE.
3
Select the appropriate mitigation command
Applying `ip ospf mtu-ignore` on the interface disables the MTU mismatch check during DBD negotiation.
This allows DBD packet processing to continue without modifying the physical or logical interface MTU values.

Key Concept

OSPF Neighbor Adjacencies and MTU Negotiation in ExStart State
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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