What's the point of prepend communities?

Steve Dodd sdodd at salesforce.com
Thu Oct 26 19:10:12 CST 2017


Keep in mind also that when you allow a customer to deprioritize a
particular peer you can really blow up your COGS model (assuming you buy
transit).

Cheers,
Steve

On Thu, Oct 26, 2017 at 1:05 PM, William Herrin <bill at herrin.us> wrote:

> On Thu, Oct 26, 2017 at 2:47 PM, Jason Lixfeld <jason+nanog at lixfeld.ca>
> wrote:
>
> > Hi Bill,
> >
> > > On Oct 26, 2017, at 2:37 PM, William Herrin <bill at herrin.us> wrote:
> > >
> > > BGP routing is based on "distance". Distance in BGP is primarily
> > calculated as the number of ASNs in the AS Path. Prepends make a path
> more
> > distance, encouraging routers to choose a different path if one is
> > available.
> >
> > I understand how prepends fit in the context of best path selection, but
> > my question was more the difference between a customer signalling the ISP
> > to prepend their AS using a BGP community stamped to a prefix vs. the
> > customer prepending their own AS instead.
> >
> >
> Hi Jason,
>
> You'd only use communities like that if you want to signal the ISP to
> deprioritize your advertisement on a particular peer or set of peers but
> not others. That's when you're getting fancy. It's not the norm. The norm
> is you want to deprioritize one of your paths as a whole. Maybe that link
> has less capacity or is enough better connected that it would always
> override your other links unless you detune it a little.
>
> I mean, you could tell the ISP to prepend everything based on a community,
> assuming they support such a community, but why would you? That needlessly
> makes things more complicated.
>
> Regards,
> Bill Herrin
>
>
> --
> William Herrin ................ herrin at dirtside.com  bill at herrin.us
> Dirtside Systems ......... Web: <http://www.dirtside.com/>
>


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