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

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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-- 
Petr Lapukhov, petr_at_INE.com
CCIE #16379 (R&S/Security/SP/Voice)
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