LevelOne GTL-2691 - Manual (Page 1443)

C HAPTER 48 | IP Interface Commands
IPv6 to IPv4 Tunnels
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The 6to4 mechanism is typically implemented almost entirely in
routers bordering between IPv4 and IPv6 domains.
The tunnel end-point address of a 6to4 tunnel is dynamically
determined by the tunnel source (local end-point node) via the IPv6
6to4 address of the packet sent from IPv6 6to4 hosts. The 6to4 end-
point address is constructed using “2002:Public IPv4 Address::/48” as
the IPv6 address prefix. This prefix can be used exactly like any other
valid IPv6 prefix, e.g., for “Neighbor Discovery for IP Version 6 (IPv6)”
defined in RFC 2461.
◆ IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
IPv4 routing topology by encapsulating them within IPv4 packets.
Tunneling can be used in a variety of ways, including the following:
■
Router-to-Router: IPv6/IPv4 routers interconnected by an IPv4
infrastructure can tunnel IPv6 packets between themselves. In this
case, the tunnel spans one segment of the end-to-end path that the
IPv6 packet takes.
■
Host-to-Router: IPv6/IPv4 hosts can tunnel IPv6 packets to an
intermediate IPv6/IPv4 router that is reachable via an IPv4
infrastructure. This type of tunnel spans the first segment of the
packet's end-to-end path.
■
Host-to-Host: IPv6/IPv4 hosts that are interconnected by an IPv4
infrastructure can tunnel IPv6 packets between themselves. In this
case, the tunnel spans the entire end-to-end path that the packet
takes; and a host can be either a 6to4 node or native IPv6 host.
■
Router-to-Host: IPv6/IPv4 routers can tunnel IPv6 packets to their
final destination IPv6/IPv4 host. This tunnel spans only the last
segment of the end-to-end path.
Tunneling techniques are classified according to the mechanism by
which the encapsulating node determines the address of the node at
the end of the tunnel. In the first two tunneling methods listed above –
router-to-router and host-to-router – the IPv6 packet is being tunneled
to a router. The end point of this type of tunnel is an intermediate
router which must decapsulate the IPv6 packet and forward it on to its
final destination. When tunneling to a router, the end point of the
tunnel is different from the destination of the packet being tunneled. So
the addresses in the IPv6 packet being tunneled can not provide the
IPv4 address of the tunnel end point. Instead, the tunnel end-point
address must be determined from information configured on the
encapsulating node. In other words, “configured tunneling” must be
used to explicitly identify the end point.
In the last two tunneling methods – host-to-host and router-to-host –
the IPv6 packet is tunneled all the way to its final destination. In this
case, the destination address of both the IPv6 packet and the
encapsulating IPv4 header identify the same node. This fact can be
exploited by encoding information in the IPv6 destination address that
will allow the encapsulating node to determine the tunnel end point
IPv4 address automatically. “6to4 automatic tunneling” employs this
technique, using an special IPv6 address format with an embedded
IPv4 address to allow tunneling nodes to automatically derive the