Hello.., you are totally right, the first reason that came to my mind is traffic engineering but there are other reasons too.

There are sometimes legitimate reasons to have a covering aggregate with some more specific announcements. Certainly there's a lot of cleanup that many should do in this area, but it might not be the best approach to this issue. 

On Tue, May 21, 2019 at 5:30 AM Alejandro Acosta <alejandroacostaalamo@gmail.com> wrote:

On 5/20/19 7:26 PM, John Kristoff wrote:
> On Mon, 20 May 2019 23:09:02 +0000
> Seth Mattinen <sethm@rollernet.us> wrote:
>
>> A good start would be killing any /24 announcement where a covering
>> aggregate exists.
> I wouldn't do this as a general rule.  If an attacker knows networks are
> 1) not pointing default, 2) dropping /24's, 3) not validating the
> aggregates, and 4) no actual legitimate aggregate exists, (all
> reasonable assumptions so far for many /24's), then they have a pretty
> good opportunity to capture that traffic.


+1 John

Seth approach could be an option _only_ if prefix has an aggregate
exists && as origin are the same


> John