What's the point of prepend communities?

Thu Oct 26 19:20:48 UTC 2017

That is exactly correct.  It is normally used to control which peers the ISP is prepended to.  Many times the ISP has a guide that shows several different communities you can use to further control BGP beyond what you normally could do yourself.

Steven Naslund
Chicago IL

-----Original Message-----
From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of William Herrin
Sent: Thursday, October 26, 2017 2:15 PM
To: Jason Lixfeld
Cc: NANOG
Subject: Re: What's the point of prepend communities?

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.

Regards,
Bill Herrin

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