6rd - IPv6 rapid deployment
6rd is a mechanism to facilitate IPv6 rapid deployment across IPv4 infrastructures of Internet service providers.
For further details please see 6rd on Wikipedia.
6rd and SixXS
There are CPE (Customer Premise Equipment) devices that support 6rd, but that do not directly support 6in4 tunnels. As 6in4, 6to4 and 6rd all use protocol-41 tunnels, one can configure such a device also as a 6to4 (using the 2002::/16 prefix) and normal static 6in4 tunnel.
For using a 6rd enabled device with SixXS, configure the IPv4 prefix length to a /32, set the remote IPv4 address to that of the PoP and configure the full IPv6 local endpoint on the device.
This should bring the tunnel up. One can then separately configure the subnet on the device to route the default subnet or a larger /48 subnet to the network behind the device.