What's the point of prepend communities?

Leo Bicknell bicknell at ufp.org
Mon Oct 30 17:24:11 CST 2017


In a message written on Thu, Oct 26, 2017 at 01:54:17PM -0600, Clinton Work wrote:
> 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

Yeah, I typoed my example, should have been:

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 2 3 5
1 2 4 5

And you just forced all traffic to the second, shorter path.  For only
customers that are dual homed to 3 & 4 without affecting other traffic.

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