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

[Tom Barron]

>>>>> On Thu, 25 Jul 2002 09:53:27 +1000, Geoff Huston <gih at telstra.net> said:

  Geoff> At 02:05 PM 7/24/2002 -0400, Andrew Partan wrote:
  >> I think we should add a note along the lines of this:
  >> An ISP may choose not to honor this community from some
  >> peers - for instace, an ISP may choose to only honor this
  >> community if sent by its customers.  An implementation
  >> should therefore provide a knob so that an ISP can drop this
  >> community from some peers.


  Geoff> I agree that this would be a  useful addition to the document

  Geoff>    Geoff

  Let me turn this conversation a bit and follow up on a hallway
conversation with Geoff.

  Irrespective of whether one honors a redistribution community
or not, one SHOULD strip this community before readvertising the
route in question.  (Of course one may decide not to readvertise,
whether because of local policy or because one is honoring redistribution
community action value 010b.)  One SHOULD strip redistribution
communities because they are *non-transitive* extended communities.

  What about ordinary, nonextended communities - which have no
"transitive" bit.  (Remember that this bit is independent of the
general transitivity of BGP attributes ....)  When should ordinary
communities be stripped and when should they be left intact?

  It seems clear to me that many communities should not be propagated
to the general Internet.  If I use communities to mark ingress points so I
can fool with the route selection process elsewhere in my iBGP domain, then
I should strip those communities on egress from my network; likewise, if I
for some reason do not strip them, a neighboring AS would be advised to
strip them on ingress.  Similarly, if my customers
signal desired local pref settings, prepends, and no-exports by sending me
communities according to a scheme that I've laid out and published, I ought
to strip these before readvertising to another AS, and another AS for whom
they have no significance would be advised to strip them on ingress if for
some reason they show up there.

  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.

  What other communities fall into this category - communities with
global significance which SHOULD be allowed to transit ASes to the
general Internet whenever the routes to which they are attached transit?

- Tom