What's the point of prepend communities?

Brett Frankenberger rbf+nanog at panix.com
Sun Oct 29 13:26:10 CST 2017

On Sun, Oct 29, 2017 at 07:01:13AM -0500, Mike Hammett wrote:
> If I understand the OP correctly, I will use this real world example: 
> https://onestep.net/communities/as174/ 
> 174:3001 through 174:3003 as compared to doing the prepending
> yourself.  What is the functional difference?
> BGP neighbors of 174 will see just as many AS hops either way, but
> non-BGP customers of 174 would see you just one hop away.  It's just
> another method of traffic engineering.

According to the link you provided, 3001..3003 are effective on "ALL
peer[s]" (which is differnet from all BGP neighbors).  So BGP-speaking
customers of 174 will not see the prepending if you use 174:3001..3003,
but peers will; but if you do the prepending yourself, then all 174's
peers and all 174's BGP-speaking customers see it.

     -- Brett

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