Question regarding anycasting in CDN setup

Charles Gucker cgucker at
Wed Feb 1 20:36:46 UTC 2012

On Wed, Feb 1, 2012 at 3:25 PM, Anurag Bhatia <me at> wrote:
> Hello everyone!
> I have a small question and was wondering if someone could help me with
> that.
> Question is - why companies like Google, Amazon are having partial
> anycasting in CDN setups? E.g if we pick a random hostname from url of
> Picasa picture - - this one is further a cname
> string and at the end you will find different A records when checked from
> different locations.

The simple answer for this is, Google cannot be expected to have a
local cache of every image supplied to them globally on every server.
 So they use unicast servers behind a DNS based geo load balancer
configuration.     As for DNS, every anycasted node is expected to be
able to resolve any DNS request that is made.

It's all a matter of disk and acceptable delay in providing the data
from the "closest" disk.


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