[dns-operations] Quick anycast primer

Patrick W. Gilmore patrick at ianai.net
Fri Jul 14 19:23:05 UTC 2006

On Jul 14, 2006, at 3:16 PM, Igor Gashinsky wrote:

> :: 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.
> Uhm, what gear do you use that doesn't do "Closest IGP metric to BGP
> next-hop" before "Lowest RID" during the best-path selection?  
> Obviously,
> your topology may be different then mine, and if you send different
> MEDs that would cause a simmilar scenario to the one you mentioned,  
> but
> then it was *your* decision to do so..  As I said, YMMV, but it works
> rather well for me (and a few million of my users)...

You aren't the first person to ask.

I used the example above because I saw it in real life.  (At least  
one other on this list can verify.)

Why the network routed the way it did is a good question.  It  
certainly was not a 'broken' network in the sense that every packet  
got where it was going.

Point is, even if you have good connectivity, anycast can still do  
strange things.


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