overlay...

 

in a galaxy far, far away... 

 

 Overlay Tunnels for IPv6

Overlay tunneling encapsulates IPv6 packets in IPv4 packets for delivery across an IPv4 infrastructure (a core network or the Internet. By using overlay tunnels, you can communicate with isolated IPv6 networks without upgrading the IPv4 infrastructure between them. Overlay tunnels can be configured between border routers or between a border router and a host; however, both tunnel endpoints must support both the IPv4 and IPv6 protocol stacks. IPv6 supports the following types of overlay tunneling mechanisms:

• Manual
• Generic routing encapsulation (GRE)
• IPv4-compatible
• 6to4
• Intrasite Automatic Tunnel Addressing Protocol (ISATAP) 

complete with pretty pictures:


http://www.cisco.com/en/US/docs/ios-xml/ios/ipv6/configuration/12-4t/ip6-tunnel.html#GUID-1E4B9CE5-E384-4AF3-B371-3DFF35034349

and this is why they included it in the topology, of course...

Overlay tunnels reduce the maximum transmission unit (MTU) of an interface by 20 octets (assuming the basic IPv4 packet header does not contain optional fields). A network using overlay tunnels is difficult to troubleshoot. Therefore, overlay tunnels connecting isolated IPv6 networks should not be considered as a final IPv6 network architecture. The use of overlay tunnels should be considered as a transition technique toward a network that supports both the IPv4 and IPv6 protocol stacks or just the IPv6 protocol stack. 

reminds one a bit of virtual-links... no point in getting in a twist about real-world scenarios...  this is what we signed on for so stop your bitching...