Re: ATOMIC AGGREGATE

[Danny McPherson <danny () tcb net>]

> I am trying to understand the BGP ATOMIC AGGREGATE attribute from a BGP
> router which receives aggregate routes. Can somebody tell me what is the
> action
> taken by a BGP router when it receives a aggregate route with ATOMIC
> AGGREGATE.
>
> I understand that the ATOMIC AGGREGATE is used when there is loss of
> information while aggregating the routes. But what does the receiver of
> the attribute do with the information. what kind of action does it take
> ?

It serves as an indication to the receiver that it can't "deaggregate" 
the prefix per some of the granularity associated with the NLRI of that
route (i.e. AS paths) may have been lost when the aggregate was created,
and deaggregation could result in the introduction of loops.

> If anyone can put some light on how this BGP attribute gets utilized in
> the real world i would greatly appreciate.

However, I don't believe many folks actually deaggregate routes in real 
networks (at least not intentionally :-).  I've seen several
implementations which don't correctly attach the attribute, though
it didn't break anything at the time.  

In the future, queuries of this nature may be more appropriately 
directed to the IDR WG list (bgp () ans net).

-danny