[nsp] bgp - aggregates and specific routes

[John Osmon]

On Wed, Jul 14, 2004 at 02:50:31PM -0400, joshua sahala wrote:
> On (14/07/04 13:34), Roger wrote:

> > I disagree.  While yes the /24 is more specific we are only advertising 
> > our aggregate /19 to upstream providers.  If the eBGP link between us 
> > and our downstream customer, using our numbers, goes down  connectivity 
> > will appear to normal because the /19 is still shown as up.
> 
>  you can disagree if you like, but routing works on the most specific
>  prefix - so /24 is more specific than /19, which is more specific
>  than /16, ad nauseum

I think that Roger wants to assert a "negative route" indicating that 
he want to take all traffic for the /19 *except for* the /24.  We just
need to reinforce the idea that it isn't necessary.  As the rest of your
message pointed out, the routing protocols will Do The Right Thing.

However, there's a corner case that might get missed:
  Roger must accept that /24 from *his* upstreams, or he'll blackhole
  traffic within his AS.

An alternative, is to setup a GRE tunnel from his border to his
customer's border, and setup a floatint static route for the /24 across
the tunnel.