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

[Jeffrey Haas]

On Thu, Jul 25, 2002 at 12:33:09PM -0500, Tom Barron wrote:
> Maybe Andrew will repost with commentary :-)

Attached to this message.

> One *could* do this, but I see no RFC or BCP that even suggests it.

But the base communities spec doesn't preclude it.

Specifically, it only talks about "these routes have common properties"
and doesn't really talk about when as <foo> is marked in there
whether it needs to be *for* as <foo> or *to* as <foo>.  Common
practice is that they are just *to* when observed outside of the AS
in question.

> I don't see
> clear consensus or documentation at least of propagation behavior for
> communities with global significance.

And aside from rules-of-thumb like Andrew Partan's (which seem to
be what people generally try to do), such behavior isn't documented
even for the ones of local 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.

If we are going to preclude an AS from marking a route with a
community with its own AS, we should:
o Make sure no one ever does this.
o Document it as a BCP (preferably with the rules of thumb)

and for vendors:
o Implement the BCP as a simple knob.

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

Unfortunately, it is common practice for some ISPs to discard all
communities, even the well-known ones, upon ingress.  This results
in unintended route leaks.

> 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?

IMO:
o Communities with global significance probably should be well-known only.
o Communities with local significance should follow Andrew's BCP
o Communities that have significance only to your immediately adjacent
  peers should use non-transitive extended communities.

> - Tom

-- 
Jeff Haas 
NextHop Technologies
-------------- next part --------------
Date: Wed, 20 Mar 2002 17:30:17 -0500
From: Andrew Partan <post-ptomaine at partan.com>
To: "Martin, Christian" <cmartin at gnilink.net>
Cc: ptomaine at shrubbery.net
Subject: Re: BGP TTL
Message-ID: <20020320173017.A9705 at partan.com>

On Wed, Mar 20, 2002 at 03:25:23PM -0500, Martin, Christian wrote:
> Perhaps communities should become mandatory transitive attributes?

That is not a good idea.  You still need to block communities you use
internally so others can't effect your router.

Ideal provider config is
	- strip all communities you use for internal markers on input
	- act on all communities you tell you customers they may use
	- strip all your communities on output
	- let all other communities thru untouched

	--asp