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Zorluk: Çok zorOSPFv2 Neighbor Adjacencies and Router ID

A network engineer is troubleshooting an OSPFv2 neighbor relationship between two routers, R1 and R2, connected via a direct GigabitEthernet0/0 interface. R1's interface is configured with an MTU of 1500 bytes. When issuing the command `show ip ospf interface gigabitethernet0/0` on R2, the engineer observes the following truncated output:

GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.0.12.2/24, Area 0
Process ID 1, Router ID 10.0.12.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
IP MTU bytes 1400

When running `show ip ospf neighbor` on R1, the neighbor status for R2 is listed as `EXSTART/ -`. Which statement correctly identifies the root cause of this stuck neighbor state and the appropriate command to bypass this requirement on R2?

  1. The neighbor is stuck in EXSTART/EXCHANGE because Database Description (DBD) packets are dropped due to the MTU size mismatch; executing `ip ospf mtu-ignore` on R2's interface allows adjacency formation.Cevap
  2. B
    The neighbor is stuck in EXSTART because the OSPF Area ID string 'Area 0' on R2 does not match R1's dotted-decimal 'Area 0.0.0.0'; executing `ip ospf 1 area 0.0.0.0` on R2's interface resolves the mismatch.
  3. C
    The neighbor is stuck in EXSTART because R2's Router Priority of 1 creates a conflict during Master/Slave negotiation; executing `ip ospf priority 0` on R2's interface forces it into Slave status.
  4. D
    The neighbor is stuck in EXSTART because R2's interface is operating in BROADCAST mode while R1 is operating in POINT_TO_POINT mode; executing `ip ospf network point-to-point` on R2's interface is required.

Cevap

The neighbor relationship remains stuck in EXSTART/EXCHANGE because R2's IP MTU (1400 bytes) does not match R1's IP MTU (1500 bytes). When OSPF routers exchange Database Description (DBD) packets in EXSTART/EXCHANGE state, each router includes its interface MTU value. If a router receives a DBD packet specifying an MTU larger than its local interface MTU, the packet is ignored, keeping the adjacency stuck in EXSTART/EXCHANGE. Configuring `ip ospf mtu-ignore` under interface configuration mode on R2 (or both routers) disables the MTU comparison check and allows the OSPF adjacency to reach FULL state.
The correct answer identifies that an IP MTU mismatch between OSPF neighbors causes adjacencies to become stuck in the EXSTART or EXCHANGE state. OSPF routers include their interface IP MTU inside Database Description (DBD) packets. When R1 sends a DBD packet with MTU 1500 and R2 receives it with MTU configured to 1400, the packet validation fails, preventing Master/Slave exchange completion. Applying the `ip ospf mtu-ignore` command under interface configuration mode ignores the MTU mismatch check in DBD packets and resolves the issue.

Adım Adım Çözüm

1
Analyze OSPF neighbor state symptoms from the scenario.
R1 sees neighbor R2 in the EXSTART/ - state.
Stuck in EXSTART state indicates successful Hello exchange and 2-Way communication, but failure during Master/Slave negotiation or Database Description (DBD) packet negotiation.
2
Compare interface IP MTU settings between R1 and R2.
R1 MTU is 1500 bytes while R2 CLI output shows `IP MTU bytes 1400`.
OSPF includes the interface IP MTU in the MTU field of DBD packets. An MTU mismatch causes the router with the smaller MTU (or the receiving router) to drop incoming DBD packets.
3
Select the appropriate Cisco IOS command to resolve an OSPF MTU mismatch.
Apply `ip ospf mtu-ignore` on the affected interface.
This command instructs OSPF to bypass checking the MTU field in incoming DBD packets, allowing neighbor adjacencies to transition to FULL state without altering the physical interface MTU.

Anahtar Kavram

OSPF Neighbor Adjacency States and MTU Mismatch Troubleshooting
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