Re: [Idr] Vendor Defaults (was Re: Review of draft-ietf-large-community-06.txt)

heasley <heas@shrubbery.net> Mon, 07 November 2016 19:11 UTC

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Date: Mon, 7 Nov 2016 19:11:06 +0000
From: heasley <heas@shrubbery.net>
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Subject: Re: [Idr] Vendor Defaults (was Re: Review of draft-ietf-large-community-06.txt)
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Mon, Nov 07, 2016 at 01:12:51PM +0100, Gert Doering:
> Hi,
> 
> On Mon, Nov 07, 2016 at 12:16:41AM +0100, Robert Raszuk wrote:
> > > The attributes changing will induce BGP churn as updates propagate.
> > > That churn may in fact trigger load-induced brown-outs and further
> > > induce churn.
> > 
> > BGP communities are not used in best path selection 
> 
> While the communities *themselves* are not, many (most? all?) major transit
> providers offer communities to influence the value local-pref is set to,
> on ingress from their customers.
> 
> So, if a customer sends 5539:70 or 5539:130 to us, this will end up 
> setting local-pref 70 or local-pref 130, and that *will* influence 
> path selection.
> 
> So, while your statement is true, it's lacking real-world consideration.

I do not understand this churn argument as it applies to large-communities.

The receiving party has to take action - ie: create policy to act upon the
large-communities - for them to affect route-selection.  There may be one
IX that is the only AS on earth that currently supports large-communities.
if large-communities are sent by default today, they only affect the size of
RIBs and size and number of BGP messages - and possibly that IX.

the only communities that take effect without adding policy are the
well-knowns and they are not present without policy to add them - and
there are no well-known large-communities.