[c-nsp] Setting "weight 255" as default for customer BGP with uRPF strict

[James]

On Sun, Nov 21, 2004 at 08:51:55PM -0600, Brian Feeny wrote:
> 
> Pete,
> 
> Ok, let me clarify, the customer is dual homed to a single ISP.  
> routers A and B both belong to the same ISP.
> 
> You can have the same "problem" though when multi-homed to two 
> different ISP's, because then the customer will announce say a /24 and 
> its aggregate /20 to ISPA and a different /24 and its aggregate to 
> ISPB.  ISPA will see the more specific /24 from ISPB, and put an entry 
> in its FIB for its egress interface.  Yet its still possible the 
> customer sends traffic to ISPA on its ingress interface......which uRPF 
> would  drop unless something such as weight was being set on the 
> neighbor.

Setting weight won't solve this problem. The more specific prefix has
next hop of egress interface, which is not the customer connected
interface. Routing management (RIB) and forwarding operations (FIB)
are always performed on the most longest match prefix, therefore
weight set on aggregate /20 won't enforce the /24 coming from another
network..

One of the ways that you can also work on with BGP customers, that scales
BETTER than manual access-lists, is to have them register all their bgp
announcements in IRR (see www.radb.net, or if you want free version, see
www.altdb.net). Then generate automatic access-list out of registered entries
from customer ASN.

But then again, if your router isn't good at filtering access-lists 
(*cough* GSR engine 0 LC, *cough*), this is challenging.

HTH,
-J

-- 
James Jun                                            TowardEX Technologies, Inc.
Technical Lead                      Boston IPv4/IPv6 Web Hosting, Colocation and
james at towardex.com            Network design/consulting & configuration services
cell: 1(978)-394-2867           web: http://www.towardex.com , noc: www.twdx.net