What's the point of prepend communities?

Clinton Work clinton at scripty.com
Thu Oct 26 19:54:17 CST 2017

I believe that Jason is asking about an ISP BGP community to prepend
their AS when the BGP routes are received from the customer (not when
advertising to a peer).   I don't see a functional difference between
the two and I suspect that ISPs added support for convenience.  If you
already send BGP communities to your ISP for other BGP route
manipulation, you can add another community for ISP AS prepending rather
than modifying your local policies to add AS prepending.   

 You are 1.
 ISP is 2.

1 1 2  - You prepend
1 2 2   - ISP prepends

Clinton Work
Airdrie, AB

On Thu, Oct 26, 2017, at 01:19 PM, Leo Bicknell wrote:
> In a message written on Thu, Oct 26, 2017 at 02:47:44PM -0400, Jason
> Lixfeld wrote:
> > I understand how prepends fit in the context of best path selection, but my question was more the difference between a customer signalling the ISP to prepend their AS using a BGP community stamped to a prefix vs. the customer prepending their own AS instead.

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