IPv6 in IPv4 Tunnel

All the IPv6 traffic is transfered into a tunnel between Barcelona and a Tatacommunications POP in Paris. This tunnel runs on the top of protocol 41 in IPv4 packets. This technology is a commonly used transition mechanism when your provider does not offer native IPv6 connectivity. The IPv6 MTU in the tunnel is 1480 bytes. We have enabled 'tunnel path mtu-discovery' on this tunnel on the Barcelona side.

The tunnel is between:

a 1841 router in Barcelona;
a POP in Paris;

More information about IPv6 over IPv4 tunnels:

IOS Documentation
Wikipedia article (very small)