[21695] in North American Network Operators' Group
Re: AS_PATH length and route selection (WAS: Strange BGP announcement)
daemon@ATHENA.MIT.EDU (Ben Black)
Wed Nov 18 14:57:47 1998
Date: Wed, 18 Nov 1998 10:39:45 -0800
From: Ben Black <black@layer8.net>
To: nanog@merit.edu
In-Reply-To: <199811180149.RAA20463@chimp.juniper.net>; from Tony Li on Tue, Nov 17, 1998 at 05:49:21PM -0800
The key to this is that the administrative preference in question has domain
scope. Assuming you are correctly implementing iBGP, whether with a full
mesh, reflectors, or confederations, and assuming consistent announcements
from external route originators, then you can't make a loop because the
same policy is in effect for all the routers in the AS.
Incomplete iBGP implementations and router-local administrative preferences
can easily be used to create loops, but that is certainly not a problem
with BGP.
Ben
On Tue, Nov 17, 1998 at 05:49:21PM -0800, Tony Li wrote:
>
> | This, of course, leads to many, many other questions (such as the
> | possibility routing loops caused by different local policies as was seen
> | with implementations of BGP4), but few of which are of a truly operational
> | issue. So I will apologize for starting a thread based on mis-information
> | and go back to my cave.
>
>
> Different local policies should all be reflected in the LOCAL_PREF, which
> should be distributed to all nodes within the domain. This should lead to
> consistent decisions within the domain and thus should avoid stable
> forwarding loops.
>
> Tony
