Regional AS model

Christopher LILJENSTOLPE cdl at asgaard.org
Thu Mar 24 20:24:03 CDT 2011


On 25Mar2011, at 09.17, Michael Hallgren wrote:

> Le jeudi 24 mars 2011 à 14:26 -0700, Bill Woodcock a écrit :
>> On Mar 24, 2011, at 1:47 PM, Patrick W. Gilmore wrote:
>>> On Mar 24, 2011, at 3:40 PM, Owen DeLong wrote:
>>>> On Mar 24, 2011, at 12:42 PM, Zaid Ali <zaid at zaidali.com> wrote:
>>>> 
>>>>> I have seen age old discussions on single AS vs multiple AS for backbone and datacenter design. I am particularly interested in operational challenges for running AS per region e.g. one AS for US, one EU etc or I have heard folks do one AS per DC. I particularly don't see any advantage in doing one AS per region or datacenter since most of the reasons I hear is to reduce the iBGP mesh. I generally prefer one AS  and making use of confederation. 
>>>> 
>>>> If you have good backbone between the locations, then, it's mostly a matter of personal preference. If you have discreet autonomous sites that are not connected by internal circuits (not VPNs), then, AS per site is greatly preferable.
>>> 
>>> We disagree.
>>> Single AS worldwide is fine with or without a backbone.
>>> Which is "preferable" is up to you, your situation, and your personal tastes. 
>> 
>> 
>> We're with Patrick on this one.  We operate a single AS across seventy-some-odd locations in dozens of countries, with very little of what an eyeball operator would call "backbone" between them, and we've never seen any potential benefit from splitting them.  I think the management headache alone would be sufficient to make it unattractive to us.

Experience with a major backbone in the early 2000's that spanned 50 core sites and 4 continents - single AS is not really a problem.  We chose IS-IS with wide metrics as the IGP, and one-layer of route-reflection for the bgp mesh control.  

The only reason I could possibly see doing multi-AS in a general case is if your route policies are different in different regions (i.e. in one region a peer AS is a 'peer' and in another region the same AS is a 'transit' or 'upstream').  You CAN do it with a single AS, but it's more painful...


>> 
>>                                -Bill
>> 
>> 
> 
> Right. I think that a single AS is most often quite fine. I think our
> problem space is rather about how you organise the routing in your AS.
> Flat, route-reflection, confederations? How much policing between 
> regions do you feel that you need? In some scenarios, I think 
> confederations may be a pretty sound replacement of the multiple-AS
> approach. Policing iBGP sessions in a route-reflector topology? Limits?
> Thoughts?
> 
> Cheers,
> 
> mh
> 
>> 
>> 
>> 
>> 
> 
> 
> 

---
李柯睿
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https://www.asgaard.org/~cdl/cdl.asc

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