Re: What's the point of prepend communities?

[Steve Dodd]

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  wrote:

> On Thu, Oct 26, 2017 at 2:47 PM, Jason Lixfeld 
> wrote:
>
> > Hi Bill,
> >
> > > On Oct 26, 2017, at 2:37 PM, William Herrin  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  her...@dirtside.com  b...@herrin.us
> Dirtside Systems . Web: 
>

Re: What's the point of prepend communities?

[Rob Foehl]

On Thu, 26 Oct 2017, Jason Lixfeld wrote:


Absolutely.  I understand the "Prepend to Network blah” use case.  The case I 
don’t get is where the ISP makes no distinction in their policy document about how 
the prepending of their own AS is applied to their upstream announcements, implying 
that it’s announced to everyone.


The "prepend to everybody" communities are usually implemented as "prepend 
to all peers", excluding customers, and for some variable definition of 
"all".  YMMV, caveat emptor, et cetera -- in most cases you'll need to 
experiment.


I've used "prepend to all peers" communities a few times to avoid 
saturating transit links that are being consolidated but still under 
contract, in order to keep the link viable and in use by the immediate 
neighbor and their customers, but otherwise make it less attractive to the 
rest of the world under normal conditions.


-Rob