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)?
Nick Olsen
Network Operations (855) FLSPEED x106
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)?
It's quite common, in my experience, that we remove (or at least filter;
usually looking at geo-origin ones only) BGP community values from peers
and filter them (modulo some set of agreed ones) from customers.
In other words, don't generally expect transitivity.
mh
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. 
Generally, the transitive BGP attribute you have the most direct control over is AS_PATH, though it's not impossible for a provider to munge the AS_PATH on routes they receive from their transits and peers, beyond your control.
Some providers might have communities that let you pass things along to their transit providers and peers, or influence traffic patterns / route propagation.
For example, if I buy transit from ISP X, and they get transit from Level3 and Sprint, they might offer a community that lets me selectively prepend to Sprint (or Level3), I can affect how traffic flows to my network. In your example, AS100 might have a community that you can set on your announcements that will cause them to set 4323:666 on that prefix when it's passed to TWTC. If they don't offer a community, then doing what you're looking for would require one of their network people to put something manual in place. Many large networks don't like to (or won't) do that because one-off requests don't scale very well, and it can add complexity when troubleshooting a connectivity problem, or when someone fat-fingers an access-list/distribute-list/prefix-list.
This varies greatly, based on the level of control your direct BGP neighbors are willing or able to offer to you. Also, in general, the farther away a network is from you (in terms of AS hops), the less likely you are to have control over how they propagate and act upon your announcements.
jms