When there is an MTU Mismatch issue in OSPF on Cisco switches, where OSPF routers have different Maximum Transmission Unit (MTU) settings on their interfaces, you may encounter log messages in the device logs that indicate the problem. These log messages can provide valuable information to help diagnose the issue. Here are some example log messages you might see in the Cisco switch logs:
- Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from FULL to DOWN, MTU mismatch Explanation: This log message indicates that the OSPF neighbor adjacency on the specified interface is transitioning from FULL to DOWN state due to an MTU mismatch. The neighboring routers have different MTU settings on their interfaces, causing the adjacency to fail.
- Message: %OSPF-4-ERRRCV: Received invalid packet: MTU mismatch from <Neighbor_IP> on <Interface> Explanation: This log message suggests that the OSPF router received an invalid OSPF packet with an MTU mismatch from the neighbor router on the specified interface. The MTU setting of the neighboring router’s interface does not match the expected value.
- Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from LOADING to EXSTART, MTU mismatch detected Explanation: This log message indicates that the OSPF neighbor adjacency on the specified interface is transitioning from LOADING to EXSTART state due to a detected MTU mismatch. The neighboring routers have different MTU settings, which is causing issues during the adjacency formation process.
- Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from EXCHANGE to DOWN, MTU mismatch found Explanation: This log message suggests that the OSPF neighbor adjacency on the specified interface is transitioning from EXCHANGE to DOWN state due to a found MTU mismatch. The MTU setting on the neighboring router’s interface does not match the expected value, leading to the adjacency failure.
One crucial requirement for successful OSPF neighbor adjacencies is having the same Maximum Transmission Unit (MTU) on their interfaces. An MTU Mismatch can prevent OSPF routers from forming adjacencies, leading to routing issues and network connectivity problems. In this article, we will explore the common causes of MTU Mismatch in OSPF and provide step-by-step solutions using Cisco commands to resolve these issues and ensure seamless OSPF operation.
- Understanding MTU Mismatch in OSPF: MTU refers to the largest packet size that a network interface can transmit without fragmentation. OSPF routers exchange OSPF Hello packets to form adjacencies. To establish these adjacencies successfully, the MTU settings on the interfaces between neighboring routers must match. If the MTU values differ, OSPF routers will not recognize each other as neighbors, leading to adjacency failures.
- Causes of MTU Mismatch: a. Configuration Errors: Incorrectly configured MTU settings on interfaces can lead to mismatch issues.
b. Interface Changes: Interface configurations that are not updated consistently across neighboring routers can cause MTU mismatches.
- Cisco Commands to Resolve MTU Mismatch:
Step 1: Access Configuration Mode Access the device’s privileged EXEC mode by entering the “enable” command. Then, enter global configuration mode with the “configure terminal” command:
Step 2: Identify OSPF Interfaces Identify the interfaces experiencing the MTU Mismatch issue:
show ip ospf interface brief
Step 3: Check MTU Configurations Verify the current MTU configurations on the problematic interfaces:
show running-config interface <Interface>
Step 4: Set Matching MTU If you find mismatched MTU values, set the same MTU on both interfaces using the following commands:
Replace “<Interface>” with the appropriate interface name and “<MTU_Value>” with the desired MTU value.
Step 5: Verify OSPF Neighbor Adjacencies Check the OSPF neighbor adjacencies to confirm that the MTU Mismatch issue has been resolved:
show ip ospf neighbor