Transitioning from IPv4 to IPv6 559
Transitioning from IPv4 to IPv6
At this point, you might be thinking that IPv6 is exciting, that it represents the future of networking,
and you want to deploy it today! Reality sets in when you realize that—although you are clearly a
networker of the future—today’s Internet still uses IPv4. How will you communicate with the
websites and e-mail servers that your business depends on?
One great solution would be to have everyone change all of their systems to IPv6 on a single day—
April 1 has been suggested. Since this seems an unlikely solution, this chapter ends with some
ideas about managing the transition period which, as a practical matter, may stretch out for years.
Success during the transition period means integrating IPv6 nodes into your network, allowing
them to communicate to IPv4 nodes, and making the whole process transparent to users.
Several transition mechanisms have been proposed, including
Dual stack
Tunneling
Translation
These transition mechanisms are described in the following sections.
Dual Stack
The dual-stack approach simply means to run IPv6 and IPv4 concurrently, with no communication
between the two. Hosts and routers have both IPv4 and IPv6 addresses and use whichever is
appropriate to reach a given resource. If a resource, such as a server, is reachable using either
protocol, IPv6 should be used.
Process 1 database summary
LSA Type Count Delete Maxage
Router 3 0 0
Network 1 0 0
Link 2 0 0
Prefix 1 0 0
Inter-area Prefix 1 0 0
Inter-area Router 0 0 0
Type-7 External 0 0 0
Type-5 Ext 0 0 0
Total 8 0 0
Example 21-8 OSPFv3 Database (Continued)
560 Chapter 21: IPv6 Routing Protocols, Configuration, and Transitioning from IPv4
To implement dual stack on a Cisco router, simply enable IPv6 and configure IPv4 and IPv6
interface addresses, as demonstrated in Example 21-9.
The dual-stack approach allows servers, clients, and applications to be gradually moved to the new
protocol. Global experience with changing applications to support IPv6 has usually resulted in
minimal impact on the applications. Furthermore, running two protocols concurrently is a well-
known and tested approach to protocol transition that has been used in the past; for example, it
was used by many organizations switching from Internetwork Packet Exchange (IPX) to IPv4 in
the 1990s.
Tunneling
Dual stack works so long as the infrastructure supports both protocols, but in some cases the core
of the network will only support IPv4. Until the core is upgraded, another technique is needed,
such as tunneling between IPv6 “islands.
With tunneling, routers that straddle the IPv4 and IPv6 worlds encapsulate the IPv6 traffic inside
IPv4 packets. The source of the IPv4 packet is the local router and the destination is the peer
router at the other end of the tunnel. When the destination router receives the IPv4 packet, it
decapsulates the external IPv4 header and forwards the enclosed IPv6 traffic.
Tunneling is effective, but decreases the maximum transmission unit (MTU) because of the
20 bytes consumed by the IPv4 header on the intermediate links. Tunneling can also be difficult
to troubleshoot.
Four types of tunneling are described in this section: manual, 6-to-4, Teredo, and Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP).
Manual Tunnels
Configuring manual tunneling is not difficult, as shown in Example 21-10 for Router A in
Figure 21-4.
Example 21-9 Dual-Stack Configuration
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66
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44
44

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