[c-nsp] BGP Multihomed Selective/Conditional Advertisement

[David Barak]

Not necessarily: the intermediary (I) could hear ATT's path as well as cogent's.  (I) could advertise their route to you through ATT, and cogent would pick this over their direct connection to you due to LocPref.  The behavior of (I) is hard to predict in advance, and it may not be fully deterministic.

Prepending toward cogent is a good idea to increase the likelihood of deterministic behavior.

-David Barak

Nathan wrote: 
> On Mon, Oct 27, 2008 at 11:56 PM,  <tkacprzynski at spencerstuart.com> wrote:
>> So what would be the behavior if I set the community for Cogent to set
>> the Local Preference to 50 in terms of transit traffic? Does that mean
>> that Cogent's originated traffic would use ATT but Cogent's peers (with
>> a shorter AS path through Cogent) would still traverse Cogent even
>> though the lower local preference is there?
> Check out "BGP Best Path Selection" on google :-)
> I think it would go like this : if Cogent has a direct connection to
> ATT, then traffic from Cogent and everywhere closer to Cogent will go
> from Cogent to ATT and then to you. If Cogent does not have a direct
> connection to ATT (OK so that is unlikely), then traffic will leave
> Cogent on a path towards ATT . . . and the intermediary might just
> send it back to Cogent . . .
> --
> HTH,
> Nathan
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