3Com MSR 50 Series Configuration Manual page 695

encapsulates an IPv4 header in the IPv6 packet and forwards it through the
physical interface of the tunnel.
3 The encapsulated packet goes through the tunnel to reach the device at the
destination end of the tunnel. The device at the destination end decapsulates the
packet if the destination address of the encapsulated packet is the device itself.
4 The device at the destination end of the tunnel forwards the packet according to
the destination address in the decapsulated IPv6 packet. If the destination address
is the device itself, the device at the destination end forwards the IPv6 packet to
the upper-layer protocol for processing.
Configured tunnel and automatic tunnel
An IPv6 over IPv4 tunnel can be established between hosts, between hosts and
devices, and between devices. The tunnel destination needs to forward packets if
the tunnel destination is not the eventual destination of the IPv6 packet.
According to the way the IPv4 address of the tunnel destination is acquired,
tunnels are divided into configured tunnel and automatic tunnel.
If the tunnel destination is not the eventual destination of the IPv6 packet, the
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device at the destination end of the tunnel (usually a router) will decapsulate
the IPv6 packet and forward it to the eventual destination after the IPv6 packet
reaches the tunnel destination. In this case, the IPv4 address of the tunnel
destination cannot be acquired from the destination address of the IPv6 packet
and it needs to be configured manually. Such a tunnel is called configured
tunnel.
If the tunnel destination is just the eventual destination of the IPv6 packet, an
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IPv4 address can be embedded into an IPv6 address so that the IPv4 address of
the tunnel destination can automatically be acquired from the destination
address of the IPv6 packet. Such a tunnel is called automatic tunnel.
Type
According to the way an IPv6 packet is encapsulated, IPv6 over IPv4 tunnels are
divided into the following types:
IPv6 manually configured tunnel
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Automatic IPv4-compatible IPv6 tunnel
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6to4 tunnel
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ISATAP tunnel
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IPv6-over-IPv4 GRE tunnel (GRE tunnel for short)
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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.
Introduction to Tunneling 695