9.2.4.2 Aggregating Routing Information

Aggregation is the process of combining the characteristics of several different routes in such a way that a single route can be advertised. Aggregation can occur as part of the decision process to reduce the amount of routing information that will be placed in the Adj-RIBs-Out.

Aggregation reduces the amount of information that a BGP speaker must store and exchange with other BGP speakers. Routes can be aggregated by applying the following procedure separately to path attributes of like type and to the Network Layer Reachability Information.

Routes that have the following attributes shall not be aggregated unless the corresponding attributes of each route are identical: MULTI_EXIT_DISC, NEXT_HOP.

Path attributes that have different type codes can not be aggregated together. Path of the same type code may be aggregated, according to the following rules:

ORIGIN attribute:

If at least one route among routes that are aggregated has ORIGIN with the value INCOMPLETE, then the aggregated route must have the ORIGIN attribute with the value INCOMPLETE. Otherwise, if at least one route among routes that are aggregated has ORIGIN with the value EGP, then the aggregated route must have the origin attribute with the value EGP. In all other case the value of the ORIGIN attribute of the aggregated route is INTERNAL.

AS_PATH attribute:

If routes to be aggregated have identical AS_PATH attributes, then the aggregated route has the same AS_PATH attribute as each individual route.

For the purpose of aggregating AS_PATH attributes we model each AS within the AS_PATH attribute as a tuple <type, value>, where "type" identifies a type of the path segment the AS belongs to (e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If the routes to be aggregated have different AS_PATH attributes, then the aggregated AS_PATH attribute shall satisfy all of the following conditions:

• all tuples of the type AS_SEQUENCE in the aggregated AS_PATH shall appear in all of the AS_PATH in the initial set of routes to be aggregated.

• all tuples of the type AS_SET in the aggregated AS_PATH shall appear in at least one of the AS_PATH in the initial set (they may appear as either AS_SET or AS_SEQUENCE types).

• for any tuple X of the type AS_SEQUENCE in the aggregated AS_PATH which precedes tuple Y in the aggregated AS_PATH, X precedes Y in each AS_PATH in the initial set which contains Y, regardless of the type of Y.

• No tuple with the same value shall appear more than once in the aggregated AS_PATH, regardless of the tuple's type.

An implementation may choose any algorithm which conforms to these rules. At a minimum a conformant implementation shall be able to perform the following algorithm that meets all of the above conditions:

• determine the longest leading sequence of tuples (as defined above) common to all the AS_PATH attributes of the routes to be aggregated. Make this sequence the leading sequence of the aggregated AS_PATH attribute.

• set the type of the rest of the tuples from the AS_PATH attributes of the routes to be aggregated to AS_SET, and append them to the aggregated AS_PATH attribute.

• if the aggregated AS_PATH has more than one tuple with the same value (regardless of tuple's type), eliminate all, but one such tuple by deleting tuples of the type AS_SET from the aggregated AS_PATH attribute.

Appendix 6, section 6.8 presents another algorithm that satisfies the conditions and allows for more complex policy configurations.

ATOMIC_AGGREGATE:

If at least one of the routes to be aggregated has ATOMIC_AGGREGATE path attribute, then the aggregated route shall have this attribute as well.

AGGREGATOR:

All AGGREGATOR attributes of all routes to be aggregated should be ignored.