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

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
>>>> 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
>>>>
>>>>
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>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>> --
>> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>>
>>
>>
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>>
>>
>
>
>

-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net