sixxs.net does not exist anymore. You want to establish an IPv6 connection to your provider using DSL?
Use FortiOS 5.4.6 or 5.6.2.
Configure your interface:
config system interface edit "wan1" set mode pppoe set allowaccess ping set type physical set role wan config ipv6 set ip6-mode pppoe set ip6-allowaccess ping set dhcp6-prefix-delegation enable set dhcp6-prefix-hint 2001:db8::/48 set dhcp6-prefix-hint-plt 900 set dhcp6-prefix-hint-vlt 10800 set autoconf enable end set username "<your username>" set password ENC <your password> set dns-server-override disable next end
This method has been tested and works in our environment for several months now.