question on BGP aggregation

Thu Oct 23 03:31:10 UTC 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,