Thanks, A further question, is it mandatory for all the aggregated information be appended at the AS path, is it possible for some aggregations do not propagate outside? By this, I mean some ASes completely conceal their aggregated ASes when propagation. thanks a lot. On Wed, Oct 22, 2008 at 10:31 PM, Ricardo Oliveira <rveloso@cs.ucla.edu> wrote:
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,