What's the point of prepend communities?

William Herrin bill at herrin.us
Thu Oct 26 19:05:25 CST 2017

On Thu, Oct 26, 2017 at 2:47 PM, Jason Lixfeld <jason+nanog at lixfeld.ca>

> 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.

Bill Herrin

William Herrin ................ herrin at dirtside.com  bill at herrin.us
Dirtside Systems ......... Web: <http://www.dirtside.com/>

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