In Cisco switches, when there is a Firewall or Access Control List (ACL) blocking OSPF traffic, you may receive the following type of log message:

%OSPF-5-ADJCHG: Process <Process_ID>, Nbr <Neighbor_IP> on <Interface> from FULL to DOWN, Firewall/ACL Blocking OSPF Traffic

Explanation of the log message:

  • %OSPF-5-ADJCHG: This log message indicates an OSPF adjacency change.
  • Process <Process_ID>: The log message includes the OSPF process ID for the specific OSPF instance running on the Cisco switch.
  • Nbr <Neighbor_IP>: This part shows the IP address of the OSPF neighbor with whom the adjacency has changed.
  • on <Interface>: The log message specifies the name of the interface where the OSPF adjacency change occurred.
  • from FULL to DOWN: This section indicates that the OSPF adjacency transitioned from the FULL state (active adjacency) to the DOWN state (inactive adjacency).
  • Firewall/ACL Blocking OSPF Traffic: The last part of the log message highlights that the issue is related to a firewall or ACL blocking OSPF traffic, which has caused the OSPF neighbor discovery to fail.

The log message serves as a notification to network administrators that a firewall or ACL is blocking OSPF traffic, preventing proper OSPF neighbor discovery. This can lead to the loss of OSPF neighbor adjacencies and disrupt OSPF routing operations. Network administrators should address the firewall or ACL configuration to allow OSPF traffic and ensure smooth OSPF neighbor discovery and routing within the network.

Solution / configuration:

when Firewalls or Access Control Lists (ACLs) inadvertently block OSPF traffic, it can lead to OSPF neighbor discovery failures, disrupting OSPF neighbor adjacencies and routing operations. In this article, we will explore the challenges of Firewall or ACL blocking OSPF traffic and provide a step-by-step solution using commands to rectify the issue on Cisco devices, enabling OSPF neighbor discovery and restoring network connectivity.

Understanding Firewall or ACL Blocking OSPF Traffic: Firewalls and ACLs are essential security measures that control network traffic. However, if they are misconfigured to filter OSPF traffic, they can prevent OSPF routers from discovering and forming neighbor adjacencies.

Solution for Resolving Firewall or ACL Blocking OSPF Traffic:

Step 1: Verify Firewall or ACL Configuration: Check the firewall or ACL configuration on each router within the network. Use the following command on each router:

show running-config | include firewall|acl

Step 2: Identify OSPF Traffic Blocked Entries: Identify the routers and interfaces where OSPF traffic is blocked by the firewall or ACL from the output of the previous command. Make a note of the entries related to OSPF traffic filtering.

Step 3: Create ACL Rule for OSPF Traffic: Based on the identified entries, create a new ACL rule to allow OSPF traffic through the firewall or ACL. Use the following command on each router:

config t
access-list <ACL_Name> permit ospf any any

Replace <ACL_Name> with a suitable name for the new ACL rule.

Step 4: Apply ACL to Appropriate Interfaces: Apply the newly created ACL rule to the appropriate interfaces to permit OSPF traffic. Use the following command on each router:

config t
interface <Interface>
ip access-group <ACL_Name> in

Replace <Interface> with the name of the router’s interface and <ACL_Name> with the name of the ACL rule created in Step 3.

Step 5: Clear OSPF Process: Clear the OSPF process on each router to apply the changes and allow OSPF routers to re-establish neighbor discovery:

clear ip ospf process

Conclusion: Resolving Firewall or ACL Blocking OSPF Traffic is essential for enabling OSPF neighbor discovery and restoring network connectivity. By following the step-by-step solution provided in this article and using the correct commands on Cisco devices, network administrators can quickly identify and rectify OSPF traffic filtering issues. Creating and applying ACL rules to permit OSPF traffic will allow OSPF routers to form neighbor adjacencies, contributing to a stable OSPF network infrastructure and efficient routing decisions. Regular monitoring and proactive management of firewall and ACL configurations will ensure a secure network environment while enabling seamless OSPF operation and reliable data exchange.

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