What's the point of prepend communities?

Thu Oct 26 19:19:43 UTC 2017

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.

Imagine:

You are 1.
ISP is 2.
ISP's peers are 3 & 4.
Your B2B partner is 5.

There are paths:

1 2 3 5
1 2 4 5

If you prepend your ASN, you get:

1 1 2 3 5
1 1 2 4 5

No difference.  If you send the ISP a prepend community for 3, you get:

1 2 3 3 5
1 2 4 5

And you just forced all traffic to the second, shorter path.

-- 
Leo Bicknell - bicknell at ufp.org
PGP keys at http://www.ufp.org/~bicknell/
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