The IPv6 support of Fortigate is very advanced. Unfortunately Fortigate does not support PPPoe with IPv6, because this is a legacy protocol. If you have a DSL connection it does work with IPv4, but not with IPv6. The workaround is: Connect yourself with IPv4 and PPPoe and request from a tunnel provider like www.sixxs.net a tunnel and later a IPv6 subnet.
The configuration on the Fortigate for the sit-tunnel looks like this:
config system sit-tunnel edit "sixxs-tun" set destination 188.8.131.52 set interface wan1 set ip6 2001:dead:babe:c5::2/64 set source 184.108.40.206 end config system interface edit "sixxs-tun" config ipv6 set ip6-allowaccess ping end end config router static6 edit 1 set device "sixxs-tun" end
The IP address 220.127.116.11 is the remote tunnel address of your POP.
The IP address 18.104.22.168 is the IP address of your external IPv4 interface.
The IP address 2001:dead:babe:c5::2/64 is the IPv6 address, you got from your tunnel provider.
Now you can use the interface sixxs-tun as your IPv6 connection and gateway.