Anycast DNS
Barry Margolin
barmar at alum.mit.edu
Wed Feb 29 15:42:36 UTC 2012
In article <mailman.58.1330527041.63724.bind-users at lists.isc.org>,
Oliver Garraux <oliver at g.garraux.net> wrote:
> On Wed, Feb 29, 2012 at 8:33 AM, takizo <paulooi at takizo.com> wrote:
> > Ju,
> >
> > What do you mean on more than one address?
> >
> > --
> > Paul Ooi
> >
> >
> >
> > On Feb 29, 2012, at 11:55 AM, ju wusuo wrote:
> >
> > Have seen some anycast DNS implementations using more than one address, some
> > times even on the same subnet, any considerations or reasons for doing
> > that?
> >
> >
>
> I assume he's asking why Google has 8.8.8.8 and 8.8.4.4, and why
> whoever runs 4.2.2.2 has 4.2.2.1, 4.2.2.2, etc. I don't have an
> answer. They may have to announce at least a /24 for BGP peers to
> accept the routes. But 8.8.8.8 and 8.8.4.4 aren't in the same /24, so
> that doesn't make sense there.
The difference is that Google is running a public DNS, while Level(3) is
an ISP and their DNS was intended just for their customers (allowing
public access is mostly a legacy of inheriting these servers from
Genuity, nee BBN Planet -- we never had a central database of all
customer address blocks from which to formulate an ACL).
So Google has to be concerned about having diverse routes from many
different ISPs, and announcing two /24's facilitates this. Level(3) is
only concerned with routing within their network, and their OSPF routing
can achieve diversity at the /32 level.
--
Barry Margolin
Arlington, MA
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