What's the point of prepend communities?

William Herrin bill at herrin.us
Thu Oct 26 19:14:38 CST 2017

On Thu, Oct 26, 2017 at 3:05 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:
> 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.

Ah, I see. I completely misread your question.

The answer you may be looking for is that the ISP can be more subtle about
which of their peers gets your prepend. Maybe you want to prefer ISP A for
your overseas traffic but prefer ISP B for your domestic traffic. ISP A can
design a community tag that causes your advertisement to be prepended but
only when sent to domestic peers.

Still not a relic but not a primary knob.

Bill Herrin

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

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