Two islands of IPv6 networks that are running ISIS (IPv6 IGP) must be connected using a tunnel
over an IPv4 network. Which tunneling method can be used to achieve this goal?

A.
GRE tunnels

B.
automatic 6 to 4 tunnels

C.
ISATAP tunnels

D.
manual tunnels (RFC 2893)

Explanation:
GRE has a protocol field that identifies the passenger protocol. GRE tunnels allow Intermediate
System-to-Intermediate System (IS-IS) or IPv6 to be specified as a passenger protocol, which
allows both IS-IS and IPv6 traffic to run over the same tunnel. If GRE did not have a protocol field,
it would be impossible to distinguish whether the tunnel was carrying IS-IS or IPv6 packets. The
GRE protocol field is why it is desirable that you tunnel IS-IS and IPv6 inside GRE
ReferencE. http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/ip6-tunnel.html