During the migration, three main classes of tools may be used to allow IPv4 to continue to work well, while supporting IPv6:
A. Dual IPv4/IPv6 stacks (dual stacks)
B. Tunneling
C. NAT Protocol Translation (NAT-PT)
A. IPv4/IPv6 Dual Stacks
The term dual stacks means that the host or router uses both IPv4 and IPv6 at the same time.
To support dual stack hosts, routers need to forward both IPv4 and IPv6 packets.
To forward IPv6 packets to the various destinations, we can use one of two general approaches:
- Native IPv6: Configure IPv6 on most or all routers, on most or all production interfaces, making all routers use a dual stack.
- IPv6 tunnels: Configure some routers with IPv6, others without IPv6, and tunnel the IPv6 packets over the IPv4 network by encapsulating IPv6 packets inside IPv4 packets.
B. Tunneling
Tunneling refers to a process by which one router or host encapsulates the IPv6 packet inside an IPv4 packet.
The networking devices forward the IPv4 packet, ignoring the fact that the packet’s payload is an IPv6 packet.
Some later device or host decapsulates the original IPv6 packet, forwarding it on to the final destination.
Point-to-Point IPv6 Tunnels
The point-to-point tunnels work like a virtual point-to-point serial link.
To create the tunnel shown in the figure, each router configures a type of virtual interface called a tunnel interface.
The configuration associated with the tunnel interfaces tells IOS the encapsulation details.
The two routers on the ends of the tunnel treat the tunnel interfaces like serial interfaces on a point-to-point serial link, at least from a Layer 3 forwarding perspective.
Point-to-Multipoint IPv6 Tunnels
Multipoint IPv6 tunnels allow the sending router–the “point” if you will–to use a single tunnel interface to send packets to multiple remote routers.
A multipoint tunnel works much like a LAN, or even more like a Non-Broadcast Multi-Access (NBMA) network like Frame Relay.
Multipoint tunnels rely on either the IPv6 packet’s destination address, or next-hop information in the IPv6 routing table, to determine which of the multiple remote devices should receive a given packet.
This decision happens dynamically on the sending router.
Summary of IPv6 Tunneling Methods
1) Manually Configured Tunnels (MCT) - STATIC - POINT-TO-POINT - Acts like a virtual point-to-point link, supporting IPv6 IGPs.
2) GRE - STATIC - POINT-TO-POINT - Generic Routing Encapsulation. Same advantages as Manually Configured, plus it can support other Layer 3 protocols over the same tunnel.
3) 6to4 - DYNAMIC - POINT-TO-MULTIPOINT - Supports global unicasts, with some extra configuration. Uses second and third quartets to store IPv4 address.
4) ISATAP - DYNAMIC - POINT-TO-MULTIPOINT - It easily supports global unicast addresses for all prefixes. Uses seventh and eighth quartets to store IPv4 address.
C. NAT Protocol Translation (NAT-PT)
At some time during the migration toward IPv6, the network may need to support the ability for an IPv4-only host to communicate with an IPv6-only host.
The IPv6 migration and coexistence RFCs actually make allowance for such a feature, but to do so, something between the two hosts must translate between the two different protocols (IPv4 and IPv6): Network Address Translation–Protocol Translation (NAT-PT).
NAT-PT translates both the source and destination IP address, translating between an IPv4 and IPv6 address for both. Not only does NAT-PT translate IP addresses, but also it translates the entire IPv4 and IPv6 header, plus other headers as well, such as TCP, UDP, and ICMP.
Note: NAT-PT has been moved to historic status (RFC 2766).
Note: More details about these topics can be found in CCNP ROUTE 642-902 Official Certification Guide, Wendell Odom, Cisco Press.
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