IPv6 over IPv4 GRE Tunnels
GRE tunnels provide two options that the other tunnel types do not—namely, encapsulating traffic
other than IPv6 and support for IPsec. Like the manually configured variety, GRE tunnels are
designed for point-to-point operation. With IPv6 as the passenger protocol, typically these tunnels
are deployed between edge routers to provide connectivity between two IPv6 “islands” across an
IPv4 cloud.
Configuring GRE tunnels for transporting IPv6 packets over an IPv4 network is straightforward.
The only difference between GRE and the manual tunneling example shown in Example 20-14 is
the syntax of the tunnel mode command, which for GRE is tunnel mode gre ipv6.