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Re: Private ASN suppression

  • From: Daniel L. Golding
  • Date: Thu May 18 10:34:14 2000


I wouldn't worry about non-confed stripping of private AS's. However the
limit of 1000 private AS's is worrisome. Some providers already use one
private AS per POP for doing confeds. With 2547 VPNs coming up (Juniper
will support this by the end of the year. I would be very surprised if
Cosine and Foundry didn't), we will need additional private AS's. The only
bright spot is that I suspect many folks wanting IP VPN services won't
want to run BGP and will do the route injections with statics. As long as
the RFC 2547 implementations support an extension to static routes so that
RD can be specificed, I suspect we'll be alright in the short term. 

>From reading 2547 I can't think of any reason that all VPN customers of a
single ISP couldn't share a single private AS, as long as they had
distinct RD numbers.

- Dan Golding

On Tue, 16 May 2000, Howard C. Berkowitz wrote:

> >I'm not sure I am looking at this the right way, but:
> >
> >It seems similar to confederations, but I'm not sure
> >the stripping method is adequate for becoming a transit AS.
> >
> >I'm guessing that this was designed for something much
> >simplier.
> >
> >The only method I could see, is that a customer is multihomed
> >to the same AS.   The customer would not likely be able to
> >obtain a AS from ARIN since they are fall under the same
> >routing policy.  The provider could strip the fake AS, and
> >announce the prefix with their AS.
> RFC1998 which assumes that AS 64750 is using address space delegated 
> to AS X, so that the AS 64750 addresses won't be explicitly 
> advertised, but only as part of
> the aggregate announced by AS X.  The AS 64750 routes should be 
> marked with the NO-EXPORT community just to make things sure.
> As you suggest, the only reason I can see to do it is if AS 64750 had 
> provider independent address space, then I could see (ignoring any 
> effects on aggregation policy), a reason for AS X to advertise the PI 
> routes.
> >
> >
> >
> >AS 64750 announces <64750> up to AS X, AS x uses it to determine
> >routing policy.  At the edge, AS X will strip it and announce just
> ><X> to the Internet as a whole.
> >
> >AS 64750 would have some control over how the traffic would enter
> >their network.  This also gives them the mechinism for failover.
> >
> >I've used this method before, but not in the same exact way.  In
> >that method, it was a smaller part of a CIDR allocation, so they
> >were aggregated into a larger block at the edge.  No reason for
> >the Internet to know about the topology when its all the same
> >AS anyway.
> >
> >So, this is only really useful if the customer has their own
> >blocks.
> >
> >For cisco to support it...someone has to be using this feature...
> Not to be flying a false flag here, I'm trying to decide if we should 
> support this feature on Nortel Versalar routers. So far, I haven't 
> seen a reason for supporting this in addition to confederations, but 
> I've been wrong before.
> A greater area of concern is managing private AS numbers given 
> widespread deployment of RFC 2547 VPNs, with the CE routers talking 
> BGP to the PE routers. For a large provider, 1K isn't a lot of AS 
> number space.  I'm inclined to believe that scalability will require 
> there must be a hierarchy of private AS numbers, essentially 
> reserving some as aggregates.
> >
> >(tm)
> >
> >  "Howard C. Berkowitz" <[email protected]>
> >     wrote
> >  >
> >  >I'm trying to understand the problem being solved by the Cisco
> >  >private AS removal feature.  In particular, what advantages does it
> >  >offer over confederations, which would seem to do the same thing when
> >  >externally advertising customer routes?  Is there a performance
> >  >benefit?
> >  >
> >  >RFC1998-style multihoming with a private AS is a possible
> >  >application, I suppose, for any routes that are NOT marked with
> >  >NO-EXPORT.