North American Network Operators Group
Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: question on BGP aggregation

  • From: Ricardo Oliveira
  • Date: Wed Oct 22 23:31:31 2008
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,