[dns-operations] Quick anycast primer

Patrick W. Gilmore patrick at ianai.net
Mon Jul 31 13:46:01 UTC 2006

On Jul 30, 2006, at 11:54 AM, Florian Weimer wrote:

> * Patrick W. Gilmore:
>> For instance, you are a very large provider who spans the globe.  You
>> see several instances of an anycast NS called XYZ.ns.
>> You are personally sitting in Washington DC.  Your network peers with
>> providers announcing XYZ.ns in DC, London, NYC, SJC, Tokyo, etc.  But
>> it turns out the guy in Tokyo has the lowest IP address, and
>> therefore won the tie-breaker since the AS_Path Length was the same
>> to all the instances.
>> Guess where you are getting your queries answered?
> Hard to tell because typical BGP decision processes prefer EBGP paths
> over IBGP paths, before the BGP ID is considered.  I guess the
> rationale for this behavior is that it's easier for the upstream AS to
> decide on the best routing path.  After all, it's closer to the
> destination and it might have better topology information (which is
> certainly true for disconnected anycast clouds).

As mentioned earlier in the thread, there are lots of things here  
that could interfere (most probable being IGP cost to next-hop).   
However, I brought up that situation because I had seen it in the  
"real world".

The point is simply that even well run networks with rich  
interconnectivity can have surprising results when dealing with anycast.

That said, I still think it's better to have at least some anycast  
than not.


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