When there is an IP Address or Subnet Mask Mismatch issue in OSPF on Cisco switches, where neighbor routers have mismatched IP addresses and subnet masks 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:

  1. Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from FULL to DOWN, Interface 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 interface mismatch. The neighboring routers have different IP addresses or subnet masks on their interfaces, causing the adjacency to fail.
  2. Message: %OSPF-4-ERRRCV: Received invalid packet: mismatched interface from <Neighbor_IP> on <Interface> Explanation: This log message suggests that the OSPF router received an invalid OSPF packet with a mismatched interface from the neighbor router on the specified interface. The IP address or subnet mask of the neighboring router’s interface does not match the expected values.
  3. Message: %OSPF-4-ERRRCV: Received invalid packet: subnet mismatch from <Neighbor_IP> on <Interface> Explanation: This log message indicates that the OSPF router received an invalid OSPF packet with a subnet mismatch from the neighbor router on the specified interface. The subnet mask of the neighboring router’s interface differs from the expected subnet mask.
  4. Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from LOADING to EXSTART, IP address mismatch Explanation: This log message suggests that the OSPF neighbor adjacency on the specified interface is transitioning from LOADING to EXSTART state due to an IP address mismatch. The neighboring routers have different IP addresses on their interfaces, preventing adjacency formation.
  5. Message: %OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from EXCHANGE to DOWN, Subnet mismatch Explanation: This log message indicates that the OSPF neighbor adjacency on the specified interface is transitioning from EXCHANGE to DOWN state due to a subnet mismatch. The neighboring routers have different subnet masks on their interfaces, causing the adjacency to fail.

These log messages provide insights into the potential “IP Address or Subnet Mask Mismatch” issue and help in identifying the root cause of OSPF adjacency problems. When troubleshooting OSPF neighbor issues related to IP addresses and subnet masks, monitoring the switch logs can be instrumental in identifying and resolving the problem effectively. It is crucial to ensure that neighbor routers have matching IP addresses and subnet masks on their interfaces to establish successful OSPF adjacencies and facilitate proper routing within the OSPF domain.

Solution:

  1. Understanding IP Address and Subnet Mask Mismatch: In OSPF, neighbor routers exchange OSPF Hello packets to form adjacencies. For successful adjacency formation, the IP addresses and subnet masks of interfaces between the neighboring routers must match. If they don’t match, OSPF will not recognize the routers as neighbors, preventing the establishment of adjacencies.
  2. Causes of IP Address or Subnet Mask Mismatch: a. Configuration Errors: Incorrectly configured IP addresses or subnet masks on interfaces can lead to mismatch issues.

b. Network Changes: Network changes or interface configurations that are not updated consistently across neighboring routers can cause mismatches.

  1. Cisco Commands to Resolve IP Address or Subnet Mask 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:

enable
configure terminal

Step 2: Identify OSPF Interfaces Identify the interfaces that are experiencing the IP Address or Subnet Mask Mismatch issue:

show ip ospf interface brief

Step 3: Check IP Address and Subnet Mask Configurations Verify the current IP address and subnet mask configurations on the problematic interfaces:

show running-config interface <Interface>

Step 4: Correct IP Address and Subnet Mask If you find mismatched IP addresses or subnet masks, modify them using the following commands:

interface <Interface>
ip address <IP_Address> <Subnet_Mask>

Replace “<Interface>” with the appropriate interface name, “<IP_Address>” with the correct IP address, and “<Subnet_Mask>” with the matching subnet mask.

Step 5: Verify OSPF Neighbor Adjacencies Check the OSPF neighbor adjacencies to confirm that the IP Address or Subnet Mask Mismatch issue has been resolved:

show ip ospf neighbor

Step 6: Save Configuration Save the configuration changes to the startup configuration to ensure they persist across device reboots.

Conclusion: Matching IP addresses and subnet masks on OSPF interfaces are essential for successful neighbor adjacency formation. By using the provided Cisco commands, network administrators can diagnose and resolve IP Address or Subnet Mask Mismatch issues effectively. Ensuring consistent IP address and subnet mask configurations across neighboring routers will facilitate the establishment of stable OSPF adjacencies, leading to efficient routing within the OSPF domain and maintaining network connectivity. Always verify the changes and test OSPF neighbor adjacencies after implementing the solutions to ensure a reliable network infrastructure.