| Tunnel Mode | Topology and Address Space | Applications |
| Automatic 6to4 | Point-to-multipoint; 2002::/16 addresses | Connecting isolated IPv6 island networks. |
| Manually configured | Point-to-point; any address space; requires dual-stack support at both ends | Carries only IPv6 packets across IPv4 networks. |
| IPv6 over IPv4 GRE | Point-to-point; unicast addresses; requires dual-stack support at both ends | Carries IPv6, CLNS, and other traffic. |
| ISATAP | Point-to-multipoint; any multicast addresses | Intended for connecting IPv6 hosts within a single site. |
| Automatic IPv4- compatible | Point-to-multipoint; ::/96 address space; requires dual-stack support at both ends | Deprecated. Cisco recommends using ISATAP tunnels instead. Coverage in this book is limited. |
| Tunnel Type | Tunnel Mode | Destination |
| Manual | ipv6ip | An IPv4 address |
| GRE over IPv4 | gre ip | An IPv4 address |
| Automatic 6to4 | ipv6ip 6to4 | Automatically determined |
| ISATAP | ipv6ip isatap | Automatically determined |
| Automatic IPv4-compatible | ipv6ip auto-tunnel | Automatically determined |
Automatic IPv4-Compatible Tunnels
The first 96 bits of the tunnel interface addresses are all 0s, and the remaining 32 bits are derived from an IPv4 address. These addresses are written as 0:0:0:0:0:0:A.B.C.D, or ::A.B.C.D, where A.B.C.D represents the IPv4 address.
Automatic 6to4 Tunnels
Per-packet basis to encapsulate traffic to the correct destination—thus its point-to-multipoint nature.
2002:border-router-IPv4-address::/48
This prefix-generation method leaves another 16 bits in the 64-bit prefix for numbering networks within a given site.
interface Ethernet2/0 description Ethernet link to the outside world ip address 10.1.100.1 255.255.255.0 ! interface Tunnel0 no ip address ipv6 address 2002:0a01:6401::1/64 tunnel source Ethernet 2/0 tunnel mode ipv6ip 6to4 ! ipv6 route 2002::/16 tunnel
ISATAP Tunnels
Treat underlying network like NBMA cloud. Point-to-multipoint operations.
[64-bit link-local or global unicast prefix]:0000:5EFE:[IPv4 address of the ISATAP link]
For example, let’s say that the IPv6 prefix in use is 2001:0DB8:0ABC:0DEF::/64 and the IPv4 tunnel destination address is 172.20.20.1. The IPv4 address, converted to hex, is AC14:1401. Therefore the ISATAP address is
2001:0DB8:0ABC:0DEF:0000:5EFE:AC14:1401
Interface MUST be configured to derive the IPv6 address using the EUI-64 method. EUI-64 addressing in a tunnel interface differs from EUI-64 on a nontunnel interface in that it derives the last 32 bits of the interface ID from the tunnel source interface’s IPv4 address.
By default, tunnel interfaces disable router advertisements (RA). However, RAs must be enabled on ISATAP tunnels to support client autoconfiguration (no ipv6 nd supress-ra).