question on BGP aggregation

Kai Chen kch670 at
Thu Oct 23 20:21:26 UTC 2008

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 at> 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:
> "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,

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