IPv6 Transition Strategies


The transition from IPv4 does not require upgrades on all nodes at the same time. Many transition mechanisms enable smooth integration of IPv4 and IPv6. Other mechanisms that allow IPv4 nodes to communicate with IPv6 nodes are available. Different situations demand different strategies. The figure illustrates the richness of available transition strategies.
Recall the advice: "Dual stack where you can, tunnel where you must." These two methods are the most common techniques to transition from IPv4 to IPv6.
Dual Stacking
Dual stacking is an integration method in which a node has implementation and connectivity to both an IPv4 and IPv6 network. This is the recommended option and involves running IPv4 and IPv6 at the same time. Router and switches are configured to support both protocols, with IPv6 being the preferred protocol.
Tunneling
The second major transition technique is tunneling. There are several tunneling techniques available, including:
Manual IPv6-over-IPv4 tunneling - An IPv6 packet is encapsulated within the IPv4 protocol. This method requires dual-stack routers.
Dynamic 6to4 tunneling - Automatically establishes the connection of IPv6 islands through an IPv4 network, typically the Internet. It dynamically applies a valid, unique IPv6 prefix to each IPv6 island, which enables the fast deployment of IPv6 in a corporate network without address retrieval from the ISPs or registries.
Other less popular tunneling techniques that are beyond the scope of this course include:
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) tunneling - Automatic overlay tunneling mechanism that uses the underlying IPv4 network as a link layer for IPv6. ISATAP tunnels allow individual IPv4 or IPv6 dual-stack hosts within a site to communicate with other such hosts on a virtual link, creating an IPv6 network using the IPv4 infrastructure.
Teredo tunneling - An IPv6 transition technology that provides host-to-host automatic tunneling instead of gateway tunneling. This approach passes unicast IPv6 traffic when dual-stacked hosts (hosts that are running both IPv6 and IPv4) are located behind one or multiple IPv4 NATs.
NAT-Protocol Translation (NAT-PT)
Cisco IOS Release 12.3(2)T and later (with the appropriate feature set) also include NAT-PT between IPv6 and IPv4. This translation allows direct communication between hosts that use different versions of the IP protocol. These translations are more complex than IPv4 NAT. At this time, this translation technique is the least favorable option and should be used as a last resort.

Cisco IOS Dual Stack
Dual stacking is an integration method that allows a node to have connectivity to an IPv4 and IPv6 network simultaneously. Each node has two protocol stacks with the configuration on the same interface or on multiple interfaces.
The dual-stack approach to IPv6 integration, in which nodes have both IPv4 and IPv6 stacks, will be one of the most commonly used integration methods. A dual-stack node chooses which stack to use based on the destination address of the packet. A dual-stack node should prefer IPv6 when it is available. Old IPv4-only applications continue to work as before. New and modified applications take advantage of both IP layers.
A new application programming interface (API) has been defined to support IPv4 and IPv6 addresses and DNS requests. An API facilitates the exchange of messages or data between two or more different software applications. An example of an API is the virtual interface between two software functions, such as a word processor and a spreadsheet. The API is built into software applications to translate IPv4 into IPv6, and vice versa using the IP conversion mechanism. New applications can use both IPv4 and IPv6.
Experience in porting IPv4 applications to IPv6 suggests that for most applications, there is a minimal change in some localized places inside the source code. This technique is well known and has been applied in the past for other protocol transitions. It enables gradual application upgrades, one by one, to IPv6.

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