Downstream Usage-BGP Communites

[ras at e-gerbil.net (Richard A Steenbergen)]

On Tue, May 10, 2011 at 05:52:39PM -0400, Nick Olsen wrote:
> Greetings NANOG,
> Was hoping to gain some insight into common practice with using BGP 
> Communities downstream.
> 
> For instance:
> We peer with AS100 (example)
> AS100 peers with TW Telecom (AS4323).
> Since I happen to know that AS100 doesn't sanitize the communities I send 
> with my routes. I can take advantage of TW Telecom's BGP communities for 
> traffic engineering. Such as 4323:666 (Keep in TWTC Backbone). Would this 
> be something that is generally frowned upon? Still under the assumption 
> that the communities aren't scrubbed off my routes. Could I do this with 
> other AS's beyond TW Telecom? Such as TW's peering with Global Crossing 
> (AS3549)?

Well first off, if you're using the words "peers with" in the normal 
sense, your routes would never propagate to AS4323 in the first place. 
Assuming what you actually mean is that at least one of those sessions 
is a transit feed, essentially all (non-stupid) networks will filter 
their own TE communities from their transits/peers, so the odds of this 
working are almost non-existant.

You also have about a 50/50 shot of AS100 stripping your communities 
before they even make it to AS4323 (or any other network). Personally my 
belief is that this is a bad thing, and you should only filter 
communities in your own name-space (i.e. $YOURASN:*), but this doesn't 
stop a large number of obnoxious networks from doing it anyways. :)

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)