Here's just a short note on IPv6 Labeled Unicast - an elegant solution to the case where you need to connect two IPv6 sites through your IPv4-only network. (The other option would, of course, be to enable IPv6 on all of your core routers, which is not always convenient).
So here I'm going to show you the minimal configuration to get this lab up and running, a trick that would make the configuration more optimal and elegant (why would you need "explicit-null"?), and take a quick look on what's going on behind the scene.
That's what my topology looks like:
