Cisco does support PPPoE with IPv6, so if your provider does support IPv6, you can do a native connection. If not, you can establish a Tunnel to SIXXS or another IPv6 provider.

ipv6 unicast-routing
interface tunnel0
  description IPv6 uplink to SixXS
  no ip address
  ipv6 enable
  ipv6 nd suppress-ra (<12.4)
  ipv6 nd ra suppress (>=12.4)
  ipv6 address [Your IPv6 Endpoint]/[Prefix Length]
  ipv6 mtu 1280 (or other MTU value)
  tunnel source [Your IPv4 Endpoint]
  tunnel destination [PoP IPv4 Endpoint]
  tunnel mode ipv6ip
!
ipv6 route 2000::/3 [PoP IPv6 Endpoint]

IPv6 does not use NAT anymore. Everytime you connect to a website in the wild, wild Internet, you have the same IP address. NAT did hide this to some extent. Now with IPv6 you can enable privacy extensions. Apple does not automatically activate them, but it is no problem to activate them permanently.

In Terminal edit the file /etc/sysctl.conf

sudo vi /etc/sysctl.conf

Don’t worry if the file does not exist, and add the following line:

net.inet6.ip6.use_tempaddr=1

Reboot your Mac. Now your Mac generates at least with every restart a temporary address.

Unfortunately Fortigate still does not support PPPoe with IPv6. 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 12.34.56.78
    set interface wan1
    set ip6 2001:dead:babe:c5::2/64
    set source 98.76.54.32
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 12.34.56.78 is the remote tunnel address of your POP.

The IP address 98.76.54.32 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.