Explanation/Reference:
Explanation:
Using the tunneling option, organizations build an overlay network that tunnels one protocol over the other by encapsulating IPv6 packets within IPv4 packets and IPv4 packets within IPv6 packets. The advantage of this approach is that the new protocol can work without disturbing the old protocol, thus providing connectivity between users of the new protocol.
Tunneling has two disadvantages, as discussed in RFC 6144:
Users of the new architecture cannot use the services of the underlying infrastructure.

Tunneling does not enable users of the new protocol to communicate with users of the old

protocol without dual-stack hosts, which negates interoperability.
Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enterprise-ipv6-solution/ white_paper_c11-676278.html