Some IPv6 client connections fail over VPN Tunnel

[christian_schindler]

Hello,

I'm using the following setup:

1 FG in primary Datacenter (Internet connection is IP4 and IPv6 capable and both are enabled)

1 FG in an office location (Internet connection is only IPv4 capable - due to provider not able to provide IPv6)

Both Sites are connected via IPSec VPN tunnel in Interface mode (via IP4). IPv6 is tunneled through the VPN.

Both Sites are configured with a Private IPv4 Address Space and a Global IPv6 Address Space.

Routing between both Sites works (both IPv4 and IPv6).

Clients in the Datacenter can access any IPv6 Host/Service on the Internet without issues.

Clients in the office location are able to connect to Websites and other Services (e.g. Outlook to Office 365, ...) but fail to connect to Skype for Business.

As soon as I disable IPv6 on one of the clients, the connection to Skype for Business works.

After taking a network trace and investigating logs on the FG, I seems that clients in the office location send requests to Skype for Business Servers, but nothing comes back. :(

To be honest I'm out of ideas...

Any thoughts on this/help would be greatly appreciated!

Thanks

Christian Schindler