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

[Tom Barron]

>>>>> On Fri, 26 Jul 2002 12:01:28 -0400, William Waites <ww at STYX.ORG> said:

  William> On Fri, Jul 26, 2002 at 10:35:30AM -0400, Jeffrey Haas wrote:
  >> 
  >> Let me give another example that might be useful from an old NANOG
  >> discussion.  AboveNet was blackholing a certain IP address in a netblock
  >> that they were otherwise cleanly routing.  This cause people who
  >> were using said route themselves grief if they were trying to get to
  >> said address.  AboveNet *could* have tacked on a community that said
  >> "this route is unclean, use at your own risk."  6461:1313 or some such. :-)

  William> I'm not sure I understand why it seems to be the common
  William> practice today to strip routes of their communities on
  William> ingres. Tagging a route with a community adds information
  William> -- I might not know the meaning of the information, but
  William> if I do there might be some way I can make use of it, as
  William> the example you give illustrates. Is it not best to try
  William> to preserve as much information as possible about a route,
  William> modulo scaling issues? 

  William> I think it can be argued that

  William> O(sum of all AS path lengths compared to number of routes) ==
  William> O(sum of all community attributes compared to number of routes)

  William> (at worst) at least as far as memory footprint goes.
  William> So I don't think there are scaling problems associated
  William> with keeping the information around.

  William> Is there a compelling reason to strip comminities off of the
  William> routes at all in the transit path?

  William> -w

As an operator, I'm motivated by a desire to write routing policy against a
known background - to match against routes containing communities that mean
something to me, either because they are of my own definition or because
they are well known communities, and not to have to worry about whether my
match clauses are true or false in the presence of communities that are
only of local significance elsewhere.

As an implementor, I have some concern about the number of communities that
can be carried in an update message, especially the number of extended
communities.  Today this probably isn't an issue, but see the commmunity
pollution example in Bruno's NANOG25 talk
(http://www.nanog.org/mtg-0206/bruno.html).

- Tom