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IPv6 uses longest-prefix match routing just like IPv4 classless interdomain routing (CIDR)
does. Many of the common routing protocols have been modified to handle longer IPv6
addresses and different header structures.
You can use and configure IPv6 static routing in the same way you would with IPv4. There
is an IPv6-specific requirement per RFC 2461 that a router must be able to determine
the link-local address of each of its neighboring routers to ensure that the target address of
a redirect message identifies the neighbor router by its link-local address. This requirement
means that using a global unicast address as a next-hop address with IPv6 routing is not
recommended.
The Cisco IOS global command to enable IPv6 is ipv6 unicast-routing.You must
enable IPv6 unicast routing before an IPv6-capable routing protocol, or an IPv6 static route,
will work.
Transitioning to IPv6 283
Routing Information Protocol next generation (RIPng) (RFC 2080) is a distance vector
routing protocol with a limit of 15 hops that uses split horizon and poison reverse to prevent
routing loops. RIPng includes the following features:
■ Is based on IPv4 Routing Information Protocol (RIP) version 2 (RIPv2) and is similar
to RIPv2
■ Uses IPv6 for transport
■ Includes the IPv6 prefix and next-hop IPv6 address
■ Uses the multicast group FF02::9, the all-RIP-routers multicast group, as the
destination address for RIP updates
■ Sends updates on UDP port 521
■ Is supported by Cisco IOS Release 12.2(2)T and later
Strategies for Implementing IPv6
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. All of these mechanisms
are applied to different situations. Figure 7-13 shows how IPv6 hosts may have to travel
across IPv4 networks during this transition.
Figure 7-13 IPv4-to-IPv6 Transition
The three most common techniques to transition from IPv4 to IPv6 are as follows:
■ Dual stack: Dual stack is an integration method in which a node has implementation
and connectivity to both an IPv4 and IPv6 network. As a result, the node and its
corresponding routers have two protocol stacks.
■ Tunneling: Several tunneling techniques are available:
— Manual IPv6-over-IPv4 tunneling: An integration method in which an
IPv6 packet is encapsulated within the IPv4 protocol. This method
requires dual-stack routers.
IPv6
Host
IPv6
Host
6to4
Router
6to4
Router
IPv6
Network
IPv4
Network
IPv6 Traffic
IPv6
A B Network
284 Chapter 7: Managing Address Spaces with NAT and IPv6
— Dynamic 6to4 tunneling: A method that automatically establishes the
connection of IPv6 islands through an IPv4 network, typically the
Internet. The 6to4 tunneling method 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.
— Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
tunneling: An 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-tohost
automatic tunneling instead of gateway tunneling. It is used to
pass unicast IPv6 traffic when dual-stacked hosts (hosts that are running
both IPv6 and IPv4) are located behind one or multiple IPv4 Network
Address Translators.
■ Proxying and translation (NAT-PT): A translation mechanism that sits between an
IPv6 network and an IPv4 network. The job of the translator is to translate IPv6 packets
into IPv4 packets and vice versa.
Dual stack is an integration method in which a node has implementation and connectivity to
both an IPv4 and IPv6 network; thus, the node has two stacks, as illustrated in Figure 7-14.
Figure 7-14 Cisco IOS Dual Stack
IPv4
IPv6
IPv4/IPv6
IPv6
Internet
IPv4
Internet
Transitioning to IPv6 285
You can accomplish this configuration on the same interface or on multiple interfaces.
Features of the dual-stack method are as follows:
■ A dual-stack node chooses which stack to use based on the destination address. A dualstack
node should prefer IPv6 when it is available. 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. 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) is defined to support both IPv4 and
IPv6 addresses and DNS requests. This new API replaces the gethostbyname and
gethostbyaddr calls. A converted application can use both IPv4 and IPv6. An
application can be converted to the new API while still using only IPv4.
■ 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.
Cisco IOS Software Releases 12.2(2)T and later are IPv6-ready. As soon as you configure
basic IPv4 and IPv6 on the interface, the interface is dual-stacked and forwards IPv4 and
IPv6 traffic on that interface. Figure 7-15 shows an example of this configuration.
Figure 7-15 Dual-Stack Configuration
Using IPv6 on a Cisco IOS router requires the global configuration command ipv6 unicastrouting.
This command enables the forwarding of IPv6 datagrams.
Tunneling is an integration method in which an IPv6 packet is encapsulated within another
protocol, such as IPv4. Figure 7-16 shows how IPv6 tunneling operates.
NOTE You must configure all interfaces that forward IPv6 traffic with an IPv6 address
using the interface command ipv6 address IPv6-address [/prefix length].
Dual-Stack
Router1
conf t
ipv6 unicast-routing
interface ethernet0
ip address 192.168.99.1 255.255.255.0
ipv6 address 3ffe:b00:c18:1::3/127
IPv6 and IPv4
Network A
286 Chapter 7: Managing Address Spaces with NAT and IPv6
Figure 7-16 IPv6 Tunneling
When IPv4 is used to encapsulate the IPv6 packet, a protocol type of 41 is specified in the
IPv4 header, and the packet has the following characteristics:
■ Includes a 20-byte IPv4 header with no options and an IPv6 header and payload.
■ Requires dual-stack routers. This process enables the connection of IPv6 islands
without the need to also convert an intermediary network to IPv6. Tunneling presents
these two issues:
— The maximum transmission unit (MTU) is effectively decreased by 20
octets if the IPv4 header does not contain an optional field.
— A tunneled network is often difficult to troubleshoot. Tunneling is an
intermediate integration and transition technique that should not be
considered a final solution. A native IPv6 architecture should be the
ultimate goal.
In a manually configured tunnel, you configure the IPv4 and IPv6 addresses statically on
the routers at each end of the tunnel. These end routers must be dual stacked, and the
configuration cannot change dynamically as network and routing needs change. You must
also properly set up routing to forward a packet between the two IPv6 networks.
Figure 7-17 illustrates the requirements for IPv6 tunnels.
Figure 7-17 IPv6 Tunnel Requirements
Dual-Stack
Router
Dual-Stack
Router
IPv6 Host IPv6 Host
IPv6
Network
IPv6
Network
IPv6 Header IPv6 Data IPv6 Header IPv6 Data
IPv4 Header IPv6 Header
Tunnel: IPv6-over-IPv4 Packet
IPv6 Data
IPv4
Network
IPv4
Dual-Stack
Router
Dual-Stack
Router
IPv4: 192.168.99.1
IPv6: 3ffe:b00:c18:1::3
IPv4: 192.168.30.1
IPv6: 3ffe:b00:c18:1::2
IPv6
Network
IPv6
Network
A B
Transitioning to IPv6 287
Tunnel endpoints can be unnumbered, but unnumbered endpoints make troubleshooting
difficult. The IPv4 practice of saving addresses for tunnel endpoints is no longer an issue
for IPv6.
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