Some Questions on BIND and IPv4 Anycast

[Bill Larson]

On Mar 9, 2007, at 7:09 PM, Barry Margolin wrote:

> In article <esrrio$2cl5$1 at sf1.isc.org>,
>  "Archimedes S. Gaviola" <agaviola at infoweapons.com> wrote:
>
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>> To Whom It May Concerned:
>>
>> Anycast address is commonly used for root nameservers for  
>> geographical
>> failover. How does BIND handles anycast address? How to determine  
>> that
>> certain IPv4 address is an anycast? Does this type of address being
>> advertised by routers on the Internet? Or just a configuration  
>> parameter on
>> the underlying operating system just like with IPv6?
>
> Anycast (in this context) is implemented simply by having multiple
> servers configured with the same IP, and advertising the route via a
> routing protocol (or having their upstream router do this for them).
> The routing protocol then arranges for client traffic to be  
> directed to
> the nearest server that advertises the address.
>
> As far as any other devices on the network can tell, there's nothing
> special about anycast -- it looks to the rest of the network as if  
> it's
> just multiple paths to the same host.  It's kind of the opposite of
> virtual hosting -- instead of one host with many IPs, it's many hosts
> with the same IP.  Of course, for management purposes the hosts also
> have their own, unique IPs.

You might want to look at http://www.isc.org/index.pl?/pubs/tn/ 
index.pl?tn=isc-tn-2004-1.txt, a Tech Note from ISC about how ISC is  
using anycast for DNS services.  Anycasting is managed completely at  
the routing level and not processed by BIND in any special manner.

Barry, your description is excellent.  Thank you.

Bill Larson