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

[William Waites]

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. :-)

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

I think it can be argued that

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

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

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

-w