10.5 Tunneling Techniques
When IPv6 development and initial deployment began in the 1990s, most of the world's networks were already built on an IPv4 infrastructure. As a result, several groups recognized that there was going to be a need for ways to transport IPv6 over IPv4 networks, and, as some people anticipated, vice versa.
One of the key reasons for tunneling is that today's Internet is IPv4-based, yet at least two major academic and research networks use IPv6 natively, and it is desirable to provide mechanisms for hosts on those networks to reach each other over the IPv4 Internet. Tunneling is one of the ways to support that communication.
As you may gather, tunneling meets a number of needs in a mixed IPv4 and IPv6 world; as a result, several kinds of tunneling methods have emerged. This section looks at several of them and examines one in detail.
10.5.1 Tunneling Overview
Tunneling, in a general sense, is encapsulating traffic. More specifically, the term usually refers to the process of encapsulating traffic at a given layer of the OSI seven-layer model within another protocol running at the same layer. Therefore, encapsulating IPv6 packets within IPv4 packets and encapsulating IPv4 packets within IPv6 packets are both considered tunneling.
For the purposes of this guide, which is to meet the CCIE Routing and Switching blueprint requirements, in this section we are mostly interested in methods of carrying IPv6 over IPv4 networks, not the other way around. This section also does not explore methods of tunneling IPv6 inside IPv6. However, you should be aware that both of these types of tunneling exist, in addition to the ones covered here. With that in mind, consider some of the more common tunneling methods, starting with a summary below:
Summary of Tunneling Methods
|
Tunnel Mode |
Topology and Address Space |
Applications |
|---|---|---|
|
Automatic 6to4 |
Point-to-multipoint; 2002::/16 addresses |
Connecting isolated IPv6 island networks. |
|
Manually configured |
Point-to-point; any address space; requires dualstack support at both ends |
Carries only IPv6 packets across IPv4 networks. |
|
IPv6 over IPv4 GRE |
Point-to-point; unicast addresses; requires dualstack support at both ends |
Carries IPv6, CLNS, and other traffic. |
|
ISATAP |
Point-to-multipoint; any multicast addresses |
Intended for connecting IPv6 hosts within a single site. |
|
Automatic IPv4-compatible |
Point-to-multipoint; ::/96 address space; requires dual-stack support at both ends |
Deprecated. Cisco recommends using ISATAP tunnels instead. |
10.5.2 Manually Configured Tunnels
This tunnel type is point-to-point in nature. Cisco IOS requires statically configuring the destination addresses of these tunnels. Configuring a manual IPv6 over IPv4 tunnel is almost identical to configuring an IPv4 GRE tunnel; the only difference is setting the tunnel mode.