CCNP BSCI Official Exam Certification Guide, Fourth Edition

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

RouterA#cc

RouterA(config)#ii

RouterA(config-if)#ii

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