# Some IPv6 connections randomly stop passing through ClusterXL in High Availability mode

**Product**: ClusterXL  
**Version**: R80.40 (EOS), R81 (EOS), R81.10 (EOS), R81.20  
**Platform**: All  
**Last Modified**: 2023-11-23

## Symptoms

- Some IPv6 connections randomly stop passing through ClusterXL in High Availability mode.
- Traffic capture on the Active cluster member shows that during the IPv6 TCP handshake, after the Active cluster member receives the [SYN-ACK] packet, the Active cluster member responds with an [ACK] packet that contains the wrong ACK sequence.

For example, Wireshark shows this wrong [ACK] packet in the "Info" column as "[TCP ACKed unseen segment]"

## Solution

This problem was fixed. The fix is included in:

- [Jumbo Hotfix Accumulator for R81.20](https://sc1.checkpoint.com/documents/Jumbo_HFA/R81.20/Default.htm) starting from Take 26
- [Jumbo Hotfix Accumulator for R81.10](https://sc1.checkpoint.com/documents/Jumbo_HFA/R81.10/Default.htm) starting from Take 110
- [Jumbo Hotfix Accumulator for R81](https://sc1.checkpoint.com/documents/Jumbo_HFA/R81/Default.htm) starting from Take 89
- [Jumbo Hotfix Accumulator for R80.40](https://sc1.checkpoint.com/documents/Jumbo_HFA/R80.40/Default.htm) starting from Take 198

If you choose not to upgrade, Check Point can supply a **Hotfix**. [Contact Check Point Support](https://www.checkpoint.com/support-services/contact-support/) to get a Hotfix for this issue.

A Support Engineer will make sure the Hotfix is compatible with your environment before providing the Hotfix.

For faster resolution and verification, please collect [CPinfo files](http://supportcontent.checkpoint.com/solutions?id=sk92739) from the Security Management Server and Security Gateways involved in the case.

**Hotfix installation instructions:**  
Refer to [sk168597 - How to install a Hotfix](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk168597).

#### NOTE

This solution has been verified for the specific scenario, described by the combination of Product, Version and Symptoms. It may not work in other scenarios.

## Article Properties

**Access Level**: General  
**Status**: Approved by TAC  
**Date Created**: 2023-05-10  
**Last Modified**: 2023-11-23