Suppose you had a set of customers than all announced to you a set of routes
and all those routes complete an aggregate
and you announce only the aggregate to those customers
and you include an AS_SET with it
then those customers will drop your aggregate, thinking there is an AS-loop
and those customers will not be able to reach each other.
An AS_SET does not prevent routing loops and can prevent correct routing.
But you must include the ATOMIC_AGGREGATE attribute, so that someone else
does not disaggregate your aggregate that does not have the AS_SET.
Regards,
Jakob.