Scope of Communities [was: Re: Last call for bgp-redistribution]

[Tom Barron]

Jeff,

  Thanks for the reply.  Some followup inline.


>>>>> On Thu, 25 Jul 2002 12:50:41 -0400, Jeffrey Haas <jhaas at nexthop.com> said:

  Jeffrey> On Thu, Jul 25, 2002 at 09:50:38AM -0500, Tom Barron wrote:
  >> It seems clear to me that many communities should not be propagated
  >> to the general Internet.

  Jeffrey> Andrew Partan posted a nice set of rules-of-thumb to this list a while
  Jeffrey> ago on this very topic

Maybe Andrew will repost with commentary :-)

  >> likewise, if I
  >> for some reason do not strip them, a neighboring AS would be advised to
  >> strip them on ingress.

  Jeffrey> That would depend.  You could leave specific communities with your AS
  Jeffrey> number on a route to indicate to the Internet that "I did *this*
  Jeffrey> to the route."

  Jeffrey> One imagined use for this is "I think this connection is congested,
  Jeffrey> I'm going to use it, but *you* might not want to".

One *could* do this, but I see no RFC or BCP that even suggests it.  Nor
do I know of operators who really do this.  What I do see is occasional
pollution by folks who inadvertently leave communities in place that were
intended only for their own use or only for use by them and their
customers.

I'm probing this area because it seems to me that historically community
attributes - unlike, say, ASPATH - have had semantics defined with fairly
local scope, but that with NOPEER (and perhaps some of the transitive
extended communities) communities may have more global reach.  I don't see
clear consensus or documentation at least of propagation behavior for
communities with global significance.

If I read you correctly, you are suggesting that even those communities
that I think have only local significance may really have meaning to the
general Internet and might ought to be preserved - more like ASPATH after
all.

  >> But certain communities not of my own definition SHOULD be kept
  >> intact and be allowed to transit my AS when I readvertise routes.
  >> Talking to Geoff in Yokohama, I think NOPEER is an an example of
  >> one of these.  If I receive a route marked with NOPEER and I choose
  >> to readvertise it (e.g. I got it from someone who pays me for transit
  >> and I'm readvertising to someone I pay for transit), then I SHOULD
  >> leave the NOPEER community intact when I readvertise.

  Jeffrey> IMO, the well-known communities (NO_EXPORT, etc.), should be left
  Jeffrey> unmolested.  

Umm, I agree except that I shouldn't be readvertising NO_EXPORT,
NO_ADVERTISE, or NO_EXPORT_SUBCONFED across an AS boundary anyway!

I'm interested in cases where the route is readvertised.  Are there other
communities than NOPEER that have global significance except those that
quash the readvertisement anyway?

- Tom