BGP Fundamentals 14 - Path Attributes Atomic_Aggregate and Aggregator

For more information on BGP, you can refer to BGP - From junior to expert.

As per RFC 4271:

ATOMIC_AGGREGATE is a well-known discretionary attribute. When a BGP speaker aggregates several routes for the purpose of advertisement to a particular peer, the AS_PATH of the aggregated route normally includes an AS_SET formed from the set of ASes from which the aggregate was formed. In many cases, the network administrator can determine if the aggregate can safely be advertised without the AS_SET, and without forming route loops. If an aggregate excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD include the ATOMIC_AGGREGATE attribute.

A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute SHOULD NOT remove the attribute when propagating the route to other speakers.

A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute MUST NOT make any NLRI of that route more specific when advertising this route to other BGP speakers.

A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute needs to be aware of the fact that the actual path to destinations, as specified in the NLRI of the route, while having the loop-free property, may not be the path specified in the AS_PATH attribute of the route.

AGGREGATOR is an optional transitive attribute, which MAY be included in updates that are formed by aggregation (see Section 9.2.2.2). A BGP speaker that performs route aggregation MAY add the AGGREGATOR attribute, which SHALL contain its own AS number and IP address. The IP address SHOULD be the same as the BGP Identifier of the speaker.


ATOMIC_AGGREGATE, has been introduced to ensure that certain aggregates are not de-aggregated. AGGREGATOR, can be added to aggregate routes to advertise which AS and which BGP speaker within that AS caused the aggregation.