a crude method of limiting long-prefix propagation

[Joel M. Halpern]

Using Exclude is better under certain topological conditions.

The primary condition is when the point where the disparate paths are 
exquivalent (in an AS sense) is such taht a reasonably large number of AS 
need the multiple advertisements

and

where topologically there are a relatively small number  of boundary AS who 
only need the aggregation, and will take of further distribution of the 
aggregated information.

The purpose is to have an alternative mechanism to limit how many AS one 
must include in the distribution list.  You or Yakov are probably in a 
better position than I to see if the topology would actually make it 
useful.  It just seems that some knob for the more remote case might well 
be helpful.  [Remote of course has nothing to do with geography.  In some 
metros, there are be several AS between facilities two blocks apart.]

Yours,
Joel M. Halpern

At 11:34 AM 3/26/01 -0500, Joe Abley wrote:
>On Mon, Mar 26, 2001 at 11:15:05AM -0500, Joel M. Halpern wrote:
> > If we are going to look down this path, for some cases at least,
> > DIST_LIST_EXCL (which lists where NOT to send the information) would also
> > seem useful.
>
>The mechanism I used in the community-based approach (and which I think
>Ben is using in his PROP_PATH attribute) is that if it looks like
>propagation control is specified for a prefix, then not mentioning an
>AS where the prefix should be announced is an implicit exclude for that
>AS.
>
>i.e. if the EPPC_ALLOW:0 community is present, and EPPC_ALLOW:A is not
>present, then the prefix must not be advertised to AS A (and must not
>be accepted by AS A if AS A understands the EPPC conventions, although
>I added that after the -01 draft was sent out).
>
>Correspondingly, I think the description Ben is working on will specify
>that if the PROP_PATH attribute is present and AS A is not present in
>the PROP_PATH, the prefix must not be advertised to or accepted by AS A.
>
>Is there a scenario where it is better to specify "I don't want this
>prefix to go to these ASes" rather than "I only want this prefix to
>go to these ASes"? I like the current logic better, I think.
>
>
>Joe