If you have an AS number mismatch in BGP configuration on a Cisco router, you might encounter a log message that indicates the issue. Here’s an example of a log message you might see:

% BGP-3-NOTIFICATION: sent to <neighbor-IP> 2/2 (open message error) 0 bytes

Explanation of the log message components:

  • % BGP-3-NOTIFICATION: This part indicates that the log message is related to BGP and specifically a BGP notification message.
  • sent to <neighbor-IP>: This portion specifies the IP address of the BGP neighbor to whom the notification was sent.
  • 2/2 (open message error): Here, 2 represents the BGP notification code for an open message error, and 2 is the subcode that provides additional details about the specific error.
  • 0 bytes: This part indicates the length of the data associated with the notification message. In the case of an open message error, the length might be minimal or zero.

In the log message example provided, the notification code 2 and subcode 2 specifically point to an open message error related to an AS number mismatch. This log message suggests that the BGP session between the local router and the specified neighbor failed to establish due to the AS number inconsistency.

To troubleshoot and resolve this issue, ensure that the AS numbers configured on both sides of the BGP peering match correctly. Verify the BGP configurations on both routers and ensure the AS numbers are consistent.

Troubleshooting Steps:

Border Gateway Protocol (BGP) is a fundamental routing protocol used in the Internet to facilitate communication between autonomous systems. One critical requirement for successful BGP peering is the consistency of Autonomous System (AS) numbers. An AS number mismatch can lead to peering issues and disruptions in network connectivity. This article guides you through the process of troubleshooting and fixing AS number mismatch in BGP.

Step 1: Identify the Issue:

  1. Check the router logs for any BGP-related errors:

show logging | include BGP-3-NOTIFICATION

Step 2: Verify BGP Configuration:

  1. Examine the AS number configuration for both your router and the neighbor’s router:

show run | section router bgp

Step 3: Confirm AS Number Mismatch:

  1. Compare the AS numbers configured on both routers. If they don’t match, that’s likely the source of the issue.

Step 4: Resolve AS Number Mismatch:

  1. Identify which AS number is incorrect and needs to be changed.
  2. Update the AS number in the BGP configuration of the router with the incorrect AS number:
  • Go to BGP configuration mode:

router bgp <incorrect-AS-number>

  •  Update the AS number to the correct value:

no router bgp <incorrect-AS-number>
router bgp <correct-AS-number>

3.Clear the BGP session to apply changes:

clear ip bgp <neighbor-IP>

4.Monitor the BGP session re-establishment:

show ip bgp summary

Conclusion: An AS number mismatch can cause disruptions in BGP peering and impact network connectivity. By following the troubleshooting steps outlined in this article and making use of Cisco commands, you can identify and rectify AS number mismatch issues in your BGP configuration. Ensuring consistency in AS numbers is vital for seamless BGP operation and maintaining efficient communication between autonomous systems. Regular checks and verification of AS numbers can prevent these issues from arising and contribute to a stable and reliable BGP network.