What's the point of prepend communities?

Thu Oct 26 19:52:07 UTC 2017

On Thu, Oct 26, 2017 at 03:05:25PM -0400, William Herrin 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.

Completely agree.  I would add that some providers' "prepend
everything" community is really "prepend to all peers" (or something
else shy of "prepend to every BGP neighbor we have").  In that case, if
the customer prepends, the prepend is seen by the provider's other
customers, but if the customer sets the "prepend to all peers"
community, the provider's customers won't see the prepend.  There are
cases where that functionality would be useful.

I have never seen a provider with a true "prepend to every BGP neighbor
we have" community, but it might well exist somewhere.

     -- Brett