Anycast DNS

[Barry Margolin]

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