What's the point of prepend communities?

Rob Foehl rwf at loonybin.net
Thu Oct 26 19:56:17 CST 2017

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 

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.


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