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Is a 6to4 tunnel inherently non-broadcast?

When you configure a 6to4 tunnel, you specify only the local IPv6 address, which has the local IPv4 address embedded in it. You do not have to specify the tunnel destination - that is done by recovering the IPv4 destination from bits 17 to 48 of the IPv6 destination. In that way 6to4 is multipoint covering the whole IPv4 address range.

So how does IPv6 multicasting work through the tunnel? Does the router map the IPv6 multicast destination onto an IPv4 multicast destination somehow? But if that is so, when the packet is decapsulated at the other end, how can the remote router recover the IPv6 multicast destination, given that there are many more IPv6 multicast addresses than IPv4.

Can anyone point me to a book or document that discusses this aspect of 6to4?

Kevin Dorrell



Re: Is a 6to4 tunnel inherently non-broadcast?

See below extract. The 6to4 Tunnel is considered to be NBMA so don't think this type of tunnel will support multicast.

An automatic 6to4 tunnel allows isolated IPv6 domains to be connected over an IPv4 network to remote IPv6 networks. The key difference between automatic 6to4 tunnels and manually configured tunnels is that the tunnel is not point-to-point; it is point-to-multipoint. In automatic 6to4 tunnels, routers are not configured in pairs because they treat the IPv4 infrastructure as a virtual nonbroadcast multiaccess (NBMA) link. The IPv4 address embedded in the IPv6 address is used to find the other end of the automatic tunnel.

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