3Com Switch 8800 Advanced Software V5 Configuration Guide

Introduction to Tunneling 377
Type
According to the way an IPv6 packet is encapsulated, IPv6 over IPv4 tunnels are
divided into the following types:
■ IPv6 manually configured tunnel
■ Automatic IPv4-compatible IPv6 tunnel
■ 6to4 tunnel
■ ISATAP tunnel
■ IPv6-over-IPv4 GRE tunnel (GRE tunnel for short)
Among the above tunnels, the IPv6 manually configured tunnel and GRE tunnel
are configured tunnels, while the automatic IPv4 compatible IPv6 tunnel, 6to4
tunnel, and intra-site automatic tunnel address protocol (ISATAP) tunnel are
automatic tunnels.
1 IPv6 manually configured tunnel
A manually configured tunnel is a point-to-point link. One link is a separate
tunnel. The IPv6 manually configured tunnel is mainly used for stable connections
requiring regular secure communication between two border routers or between a
border router and a host, or for connections to remote IPv6 networks.
2 Automatic IPv4-compatible IPv6 tunnel
An automatic IPv4-compatible IPv6 tunnel is a point-to-multipoint link.
IPv4-compatible IPv6 addresses are adopted at both ends of such a tunnel. The
address format is 0:0:0:0:0:0:a.b.c.d/96, where a.b.c.d represents an embedded
IPv4 address. The tunnel destination is automatically determined by the embedded
IPv4 address, which makes it easy to create a tunnel for IPv6 over IPv4. However,
an automatic IPv4-compatible IPv6 tunnel must use IPv4-compatible IPv6
addresses and it is still dependent on IPv4 addresses. Therefore, automatic
IPv4-compatible IPv6 tunnels have limitations.
3 6to4 tunnel
A 6to4 tunnel is a point-to-multipoint tunnel and is used to connect multiple
isolated IPv6 domains over an IPv4 network to remote IPv6 networks. The
embedded IPv4 address in an IPv6 address is used to automatically acquire the
destination of the tunnel. The automatic 6to4 tunnel adopts 6to4 addresses. The
address format is 2002:abcd:efgh:subnet number::interface ID/64, where
abcd:efgh represents the 32-bit source IPv4 address of the 6to4 tunnel, in
hexadecimal notation. For example, 1.1.1.1 can be represented by 0101:0101.
The tunnel destination is automatically determined by the embedded IPv4 address,
which makes it easy to create a 6to4 tunnel.
Since the 16-bit subnet number of the 64-bit address prefix in 6to4 addresses can
be customized and the first 48 bits in the address prefix are fixed by a permanent
value and the IPv4 address of the tunnel source or destination, it is possible that
IPv6 packets can be forwarded by the tunnel. A 6to4 tunnel interconnects IPv6
networks and overcomes the limitations of an automatic IPv4-compatible IPv6
tunnel.