What's the point of prepend communities?

Naslund, Steve SNaslund at medline.com
Thu Oct 26 20:22:36 CST 2017

No, Mr Herrin was correct in the first place.  The BGP communities are used to control to which peers the prepend is applied.  You cannot do this within your own BGP configuration.  Here is the use case.

I have a connection to AT&T and I have one to Comcast

I want AT&T customers to come to me over the AT&T circuit.

I want everyone else to come to me over Comcast.

If I just prepend my own AS onto the AT&T connection I might actually cause AT&T customers to reach me via Comcast (shorter AS path potentially).  If I want to prevent that I send a prepend community to AT&T which tells them to prepend toward their peers BUT NOT WITHIN THEIR OWN NETWORK.  There are usually multiple choices here to affect different subsets of their peers (customers, peer carriers, etc).  There is also the case where I have multiple AT&T connections.  If I prepend just one of them the way you suggest, all of my traffic will probably come in the other connection.  If I prepend using communities I can get some traffic (from AT&T peers) on one connection and get most of the transit on the other connection.

What you are doing in effect is making your connection with AT&T look like AT&T is a peer rather than a transit AS.  You can also do this to more selectively influence your inbound traffic when you are multihomed to different carriers (just a simple prepend like you suggest may not be granular enough, I have seen cases where a single prepend pushes almost all your traffic to your other carrier).

The old way to do this was to ask your ISP to treat you like a peer rather than a transit customer.  Now you can control that yourself just by setting the correct communities, it can also be programmatically controlled.  Picture you have the configuration like I talked about above with AT&T and Comcast but now Comcast is overloaded or impaired.  By changing your communities you can reroute most of the traffic to AT&T without a MACD request to them.

Steven Naslund
Chicago IL

-----Original Message-----
From: NANOG [mailto:nanog-bounces+snaslund=medline.com at nanog.org] On Behalf Of Clinton Work
Sent: Thursday, October 26, 2017 2:54 PM
To: nanog at nanog.org
Subject: Re: What's the point of prepend communities?

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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