BGP aggregator and atomic aggregate attributes

Atomic Aggregate: to inform other routers that certain prefixes were aggregated.

The atomic aggregate is a well-known discretionary attribute. It can be set by a router that aggregates prefixes. When it is set, it is used to alert other BGP speakers that the route is an aggregation, that some attributes may be lost and that the route should not be made into more specific routes.

Aggregator: attribute that may be included in Updates sent by routers performing aggregation.

The aggregator is an optional transitive attribute. To have a BGP speaking router inform peers that it is performing route aggregation, the aggregator attribute may be attached to an Update message. The aggregator contains the BGP speakers IP address and AS number. The aggregator can also choose to set the atomic aggregate.

In Junos, the atomic aggregate and the aggregator are not attached by default. Both have to be specified under the aggregate routes inside the ‘as-path’ options.