Re: OT: Building Scalable Data Centers: BGP is the Better IGP

One general question I have regarding Clos fabrics is how you deal with so
many equal cost paths? In some of the topologies I see have maybe 32 or
even 128 spine switches giving you say 32 equal cost paths but most routing
protocols usually only support 6,8, or 16 max paths.

Can anyone explain this to me?

On Tue, Feb 26, 2013 at 4:53 AM, Carlos G Mendioroz <tron_at_huapi.ba.ar>wrote:

> Ah, ok, now I see what you mean.
> You do have independent Clos fabrics (much like storage A/B but
> potentially C/D/... :) and then if one of the spines loose a destination,
> because of default route it would continue to receive traffic to it.
> Thanks.
>
>
> Petr Lapukhov @ 26/02/2013 04:34 -0300 dixit:
>
> The problem is that in "classic" folded Clos topology there is only a
>> single L3 path from "middle stage" (spine) to the input/output stage
>> (leaf or ToR). Therefore, if a single link b/w leaf and spine fails,
>> there is no other way around from that spine to that leaf.
>>
>> Imagine that you announce a default route to a leaf from spine device,
>> and on that same spine another link to a different leaf fails. The
>> first leaf switch would not know about the failure, since it only
>> receives a default route, and will keep sending packets even to the
>> spine with the failed link, obliviously following all ECMP paths -
>> thus effectively black-holing traffic.
>>
>> If you want to allow for route summarization in Clos topologies you
>> need to make sure there is at least two parallel paths from spine to
>> leaf (by "compressing" the spine devices and mapping multiple links
>> from a leaf on the same spine device). This would make you resilient
>> to a single link failure, but would expose to a different problem -
>> when one of the parallel paths fail, the other one will have to pick
>> up 2x the traffic, often creating congestion. There is always a
>> tradeoff you have to make...
>>
>> 2013/2/25 Carlos G Mendioroz <tron_at_huapi.ba.ar>:
>>
>>> Interesting :)
>>> Petr, can you please give me some hint on how default route only can led
>>> to
>>> black holing ? (Slide 24). I fail to see how "default only" where by
>>> definition there are no details can create a hole.
>>>
>>> Thanks,
>>> -Carlos
>>>
>>> Petr Lapukhov @ 23/02/2013 16:48 -0300 dixit:
>>>
>>> There is even more fun when you add centralized routing control there,
>>>> doing SDN-type stuff with BGP only :)
>>>>
>>>> 2013/2/23 Antonio Soares <amsoares_at_netcabo.pt>:
>>>>
>>>>>
>>>>> I found this presentation made by Petr Lapukhov:
>>>>>
>>>>>
>>>>> http://www.nanog.org/meetings/**nanog55/abstracts.php?pt=**
>>>>> MTk0MiZuYW5vZzU1&nm=n<http://www.nanog.org/meetings/nanog55/abstracts.php?pt=MTk0MiZuYW5vZzU1&nm=n>
>>>>> anog55
>>>>>
>>>>> BGP to the ToR. No OSPF, no vPC, no L2. Really excelent.
>>>>>
>>>>>
>>>>>
>>>>> Regards,
>>>>>
>>>>> Antonio Soares, CCIE #18473 (R&S/SP)
>>>>> amsoares_at_netcabo.pt
>>>>> http://www.ccie18473.net
>>>>>
>>>>>
>>>>> Blogs and organic groups at http://www.ccie.net
>>>>>
>>>>> ______________________________**______________________________**
>>>>> ___________
>>>>> Subscription information may be found at:
>>>>> http://www.groupstudy.com/**list/CCIELab.html<http://www.groupstudy.com/list/CCIELab.html>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>> --
>>> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>>>
>>>
>>>
>>> Blogs and organic groups at http://www.ccie.net
>>>
>>> ______________________________**______________________________**
>>> ___________
>>> Subscription information may be found at:
>>> http://www.groupstudy.com/**list/CCIELab.html<http://www.groupstudy.com/list/CCIELab.html>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>
>>
>>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>
>
> Blogs and organic groups at http://www.ccie.net
>
> ______________________________**______________________________**
> ___________
> Subscription information may be found at: http://www.groupstudy.com/**
> list/CCIELab.html <http://www.groupstudy.com/list/CCIELab.html>
>
>
>
>
>
>
>
>

-- 
Marc Abel
CCIE #35470
(Routing and Switching)
Blogs and organic groups at http://www.ccie.net