A BGP speaker may transmit routes with overlapping Network Layer Reachability Information (NLRI) to another BGP speaker. NLRI overlap occurs when a set of destinations are identified in non-matching multiple routes. Since BGP encodes NLRI using IP prefixes, overlap will always exhibit subset relationships. A route describing a smaller set of destinations (a longer prefix) is said to be more specific than a route describing a larger set of destinations (a shorted prefix); similarly, a route describing a larger set of destinations (a shorter prefix) is said to be less specific than a route describing a smaller set of destinations (a longer prefix).
The precedence relationship effectively decomposes less specific routes into two parts:
When overlapping routes are present in the same Adj-RIB-In, the more specific route shall take precedence, in order from more specific to least specific.
The set of destinations described by the overlap represents a portion of the less specific route that is feasible, but is not currently in use. If a more specific route is later withdrawn, the set of destinations described by the overlap will still be reachable using the less specific route.
If a BGP speaker receives overlapping routes, the Decision Process shall take into account the semantics of the overlapping routes. In particular, if a BGP speaker accepts the less specific route while rejecting the more specific route from the same peer, then the destinations represented by the overlap may not forward along the ASs listed in the AS_PATH attribute of that route. Therefore, a BGP speaker has the following choices:
If a BGP speaker chooses 5), then it should add ATOMIC_AGGREGATE attribute to the route. A route that carries ATOMIC_AGGREGATE attribute can not be de-aggregated. That is, the NLRI of this route can not be made more specific. Forwarding along such a route does not guarantee that IP packets will actually traverse only ASs listed in the AS_PATH attribute of the route. If a BGP speaker chooses 1), it must not advertise the more general route without the more specific route.