Tag Archives: tunnel


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.


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
 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 The IPv4 address, converted to hex, is AC14:1401. Therefore the ISATAP address is


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).