Tunnelling, the second method of integrating IPv6 and IPv4 networks, entails encapsulating the IPv6 packets or datagrams and sending them over IPv4 networks. In order to support the different tunnelling mechanisms that will be described in this section, Cisco IOS edge routers must have a dual-stack implementation that allows the IPv6 packets to be encapsulated in IPv4 packets, and then de-encapsulated at the terminating router. It should be noted that intermediate routers do not need to run IPv6. In other words, these routers would simply be IPv4-only routers. Figure 8.1 below illustrates a typical tunnelling implementation:
Figure 8.1 – Tunnelling IPv6 Packets over IPv4 Networks
Referencing Figure 8.1, assuming that IPv6 Host 1 is sending datagrams to IPv6 Host 2, the following sequence of events occurs as those packets transit the network:
- IPv6 Host 1 sends the IPv6 packets destined to IPv6 Host 2 to its default gateway, which is router R4. These are native IPv6 packets, with IPv6 source and destination addresses included in the header.
- Router R4 is a dual-stack router. The LAN interface has been enabled for IPv6, while the WAN interface has been enabled for IPv4. R4 has a tunnel configured between its WAN interface and the WAN interface of R1, which is also a dual-stack router. Upon receiving the IPv6 packets, R4 encapsulates them in IPv4 packets and forwards them to R2. The destination address for these packets is sent to R1 and the router sets the value of the IPv4 header to 41 to indicate encapsulation of IPv6 packets in IPv4 packets.
- R2 receives the IPv4 packets and simply routes or forwards those towards their final destination using the destination address specified in the IPv4 header.
- R3 receives the IPv4 packets from R2 and simply forwards those towards their final destination using the destination address specified in the IPv4 packet header.
- R1, the terminating router and exit point for the tunnel, receives the native IPv4 packets and de-encapsulates them, leaving only the IPv6 datagrams. The router then forwards the IPv6 packets to Host 2.
The encapsulation and de-encapsulation process is transparent to the two hosts, as well as to the intermediate routers between the tunnel endpoints. Several methods can be used to tunnel IPv6 packets in IPv4 packets (see below). We won’t go into any configuration details in this guide because it’s beyond the CCNA exam requirements.
Other tunnelling methods include the below. Cisco may expect you know they exist but you should not be asked any questions about how they operate.
- Static (manually configured) IPv6 tunnelling
- 6to4 tunnelling
- Automatic IPv4-compatible tunnelling
- ISATAP tunnelling
- Generic Routing Encapsulation tunnelling