Question about 6to4

Steve Wilcox stevewilcox at
Thu May 14 23:49:20 CEST 2009

Hey Ted,
 you sort of answered your own question - the aggregate is announced as an
anycast and the v6 routing table doesn't know where the individual v6
packets are destined.

So the v6 packets find their way to the nearest 6to4 relay, that then
converts to v4 and its routed out as v4. As you say HE is one major sink for
that, as a result of them being so well connected.

Thats one of the downsides with 6to4 - the packet may go in the wrong
direction in v6 before passing through the relay and then heading in the
opposite direction to find the v4 endpoint.


On Thu, May 14, 2009 at 10:32 PM, Ted Mittelstaedt <tedm at> wrote:

> Hi All,
>  I apologize in advance if this has been asked before a million times but
> I have what is probably a stupid question about 6to4.
>  We are in process of connecting to native IPv6, I am currently getting
> the IPv6 BGP table from our upstream.
>  I see that Linksys is supporting IPv6 out-of-the-box in some of it's
> routers, via 6to4
>  I figured this might be useful for some of our customers, to setup
> a 6to4 relay router for these Linksys devices to use.  I have found plenty
> of info on the Internet to setup a router as a 6to4 relay.
>  My question concerns how exactly 6to4 -works-
>  My understanding is that RFC3068 defines 2002:c058:6301:: as the
> anycast for the (in this case) customer router to find the 6to4 relay I
> create.  And that a 6to4 relay then uses a manufactured 2002:: IPv6
> address formed by using it's IPv4 address with the 2002:: prefix, and
> that any 6to4 routers tunneled into it are using their manufactured
> 2002:xxxx:: addresses.
>  So, if this is the case then wouldn't every 6to4 relay that's advertising
> on the Intenet be present in the IPv6 BGP table?
>  In looking at the various IPv6 looking glasses on the Internet, I see
> a handful of 2002:: routes out there.
>  But there seems to be no consistency anywhere.  Most of the looking
> glasses
> seem to show Hurricaine Electric's 2002:: advertisement, including my
> own table.
>  But beyond that, the advertisements for other relays seem to exist in
> one router, but not in others.
>  Advertisements for native routes do seem to be consistent.
>  How exactly do routers know where to forward IPv6 packets destined for
> a given 6to4   2002:: address if there is no route in their table?  And if
> they are just sending 6to4 traffic to HE then how does HE know what to
> do with it if it's not for their network?
> Thanks,
> Ted

Network Operations - Standards & Design
Google Inc.
E: stevewilcox at
M: +44 7920 041930
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