Consistent routing policy?

Mon Sep 16 19:04:08 UTC 2019

For any router which receives both announcements, longest match always wins over all other BGP tie-breaking criteria.

This is almost always summarized as “Longest Match always wins” because virtually any engineer recognizes that the
winner is selected only from the available contestants, not from unknown distant contestants not present at the router
in question.

Owen

> On Sep 16, 2019, at 11:26 , Töma Gavrichenkov <ximaera at gmail.com> wrote:
> 
> Peace,
> 
> On Mon, Sep 16, 2019, 6:06 AM Mark Tinka <mark.tinka at seacom.mu <mailto:mark.tinka at seacom.mu>> wrote:
> Longest match always wins... so provided your /22's are in the global
> table, traffic will follow the path toward them before the /21 is preferred.
> 
> Not always.
> 
> E.g. imagine an ISP who has two connections to the outside world: one through a major ISP and the other through an IX.
> 
> Such an ISP would be quite inclined both financially and technically to import routes just from the IX and set the default to the upstream ISP link.  Therefore, an advertisement from the IX would always win no matter the length.
> 
> There are more complicated cases as well.
> 
> --
> Töma

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