[nsp] bgp - aggregates and specific routes

[joshua sahala]

On (14/07/04 13:34), Roger wrote:
> >
> Yes - the 192.168.16.0/24 falls within the 192.168.0.0/19 range.  This 
> was mentioned in the previous post.  The /24 is downstream from us and 
> they're getting their own ASN. 
> 
> In real life we have a /19 delgated to us by ARIN.

 ok - that makes sense
 
> >how are you learning the /24?  if you are learning it from your
> >customer, then when the link goes down, you will stop learning that
> >prefix and will subsequently stop advertising it.  if you are
> >learning it via some other means, then i'd need to know how that is
> >to answer this.
> > 
> >
> We are learning the /24 via eBGP.  Now if the eBGP session between us 
> and said downstream customer(16.0/24) goes down connectivity to the 
> 16.0/24 will still occur because it will be lumped in w/ our 
> aggregate...  We need to prevent this when the eBGP session goes down. 

 prevent what?  you will no longer learn the /24, so you will no
 longer advertise the /24, only the /19.
 
> >/24 is more specific than /19, so for addresses in that /24, traffic
> >will go towards your customers other providers.  all other traffic
> >for the /19 will come to you
> >
> 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'm looking for a route-map statement or some conditional advertisement 
> statement.

 you don't need anything conditional, so long as they are advertising
 the /24 to another provider, the traffic for the addresses in that
 block WILL go to them.  why the desire to complicate things? 
 
> No - the problem is we WANT to be a transit AS for our downstream 
> customer 16.0/24 only if a eBGP link between us and them is in working 
> order.  I'll continue to look.

 if they are advertising the /24 to you, and you are in turn
 advertising that /24 to your providers, you will.  if they stop
 advertising the /24, you will stop advertising the /24
 
hth

/joshua
-- 
A common mistake that people make when trying to design something 
completely foolproof is to underestimate the ingenuity of complete
fools.
	- Douglas Adams -

-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: Digital signature
Url : https://puck.nether.net/pipermail/cisco-nsp/attachments/20040714/0eeb81e4/attachment.bin