[108735] in North American Network Operators' Group
Re: question on BGP aggregation
daemon@ATHENA.MIT.EDU (Ricardo Oliveira)
Wed Oct 22 23:31:44 2008
In-Reply-To: <e81393830810222017n164e61dfw831d50dd812f0688@mail.gmail.com>
From: Ricardo Oliveira <rveloso@cs.ucla.edu>
Date: Wed, 22 Oct 2008 20:31:10 -0700
To: Kai Chen <kch670@eecs.northwestern.edu>
Cc: nanog@merit.edu
Errors-To: nanog-bounces@nanog.org
the ASes in the AS_SET resulted from merging 2 or more AS_PATHS, you
only know at least one of them is connected to AS3 ...
more details at rfc4271:
http://www.ietf.org/rfc/rfc4271.txt
"An AS_SET implies that the destinations listed in the NLRI can
be reached through paths that traverse at least some of the
constituent autonomous systems. AS_SETs provide sufficient
information to avoid routing information looping; however,
their use may prune potentially feasible paths because such
paths are no longer listed individually in the form of
AS_SEQUENCEs. In practice, this is not likely to be a problem
because once an IP packet arrives at the edge of a group of
autonomous systems, the BGP speaker is likely to have more
detailed path information and can distinguish individual paths
from destinations.
"
--Ricardo
On Oct 22, 2008, at 8:17 PM, Kai Chen wrote:
> Hi,
>
> I observe some BGP AS paths collected from Routeview having the AS-set
> in the last hop. According to my understanding, this is BGP route
> aggregation. However, my question is as follows:
> Suppose, there is a path AS1 AS2 AS3 {AS4 AS5 AS6}, how AS4 AS5 AS6
> connect to AS3?
> Does it necessarily mean that AS4 AS5 AS6 are direct neighbors of AS4,
> and AS4 aggregate the routes from AS4 AS5 AS6 then exporting outside.
> or, it could be other cases such as AS4 aggregate AS5 AS6 first, and
> then AS3 aggregate AS4?
>
> Thanks in advance,
