[dns-operations] Anycast vs. unicast NS
michael at rancid.berkeley.edu
Mon Mar 21 03:16:54 UTC 2011
On 03/18/11 09:57, Dobbins, Roland wrote:
> On Mar 18, 2011, at 11:28 PM, Joe Abley wrote:
>> While it's philosophically feasible that there is some
>> anycast-specific failure mode that might afflict both together,
>> which might lead people to think that the use of anycast is a
>> single point of failure, I'll observe that I have never heard of
>> such a failure to date.
> Concur. The use of multiple anycast addresses pretty much takes care
> of any scenario I can come up with, at any rate.
Moreover, it is possible to engineer anycast so that if all of the
anycast routing goop and healthchecks fail, it essentially falls back to
being unicast. Unicast can be made to be the worst-case-scenario for
This is why I find the debate between "anycast" and "unicast" to be a
false dichotomy. It's really just a debate about design and operational
procedures. Anycast can be engineered to avoid common-mode failures**
as well as anycast+unicast. We're just incorrectly superimposing
technology over a design debate.
**Common-mode failure is a more apt concept for what we're discussing
than single-point of failure, and it is a fairly common concept in the
literature in complex systems, normal accidents, and nonlinear dynamics.
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