FRITZ!Box behind pfSense - Prefix Delegation

wilixje

I have an issue with deploying IPv6 to my FRITZ!Box 7360.

For IPv6 connectivity I'm using a SixXS tunnel because my ISP isn't offering a native IPv6 connection.
The pfSense is used as firewall and router for the internet connection, directly connected to the ISP modem.

ISP –>> Modem -->> pfSense -->> Bridged interface -->> FRITZ!Box 7360 for ATA and Wi-Fi
ISP -->> Modem -->> pfSense -->> Bridged interface -->> Switch for network

The IPv6 subnet is working fine for my clients, except the FRITZ!Box. The FRITZ!Box is configured as:
Internet service provider -> Existing connection over LAN

When I choose for the option External Modem or Router the FRITZ!Box will disable and hide the IPv6 settings.

In the logfile of the FRITZ!Box showed that IPv6 is working for a few seconds (I replaced some parts in the log output with xxxx):
11.02.16 12:32:09 Could not establish IPv6 Internet connection: No response from DHCPv6 server (SOL).
11.02.16 12:31:53 IPv6 Internet connection established successfully. IP address: 2001:610:xxxx:xxxx:3631:c4ff:fe22:5d02
11.02.16 12:31:52 Internet connection established successfully. IP address: 192.168.1.251, DNS server: 192.168.1.254 and 192.168.1.254, Gateway: 192.168.1.254

The pfSense DHCPv6 server is configured with the following settings:

The additional log details of the FRITZBOX
Could not establish IPv6 Internet connection: No response from DHCPv6 server (SOL).

This is an event message from the "Internet Connection" area.
This message notifies you that the DHCPv6 server of the provider does not respond to the FRITZ!Box's initial request (solicitation) to establish an IPv6 Internet connection.

Check your IPv6 connection settings in the "Internet / Account Information / IPv6" menu.
Make sure that these settings match the information from your Internet service provider.
If the message continues to occur in subsequent attempts, contact your Internet Service Provider.

I assume that I need to request a /56 subnet at SixXS to make a correct prefix delegation. But I would like to be sure that this is the issue before requesting a second subnet. So maybe someone here had some experience with prefix delegation or using a FRITZ!Box behind pfSense with IPv6 enabled.