Route Summarization

On medium or large-scale Border Gateway Protocol (BGP) networks, the BGP routing table on a device contains a large number of routing entries. Storing the routing table consumes a great deal of memory, and transmitting and processing routing information consume significant network resources. Route summarization can reduce the size of a routing table, prevent specific routes from being advertised, and minimize the impact of route flapping on network performance.

Route summarization is the process of summarizing specific routes with the same IP prefix into a summary route. BGP supports automatic and manual route summarization. Table 1 defines the differences between the two modes.

**Table 1** Differences between automatic and manual route summarization
Route Summarization Mode	Implementation	Characteristics
Automatic route summarization	After automatic route summarization is configured, BGP summarizes routes based on the natural network segment and sends only the summarized route to peers. For example, 10.1.1.1/24 and 10.2.1.1/24 are summarized into 10.0.0.0/8, which is a Class A address.	BGP summarizes only local routes that are imported using the import-route command. During BGP route selection, an automatically summarized route has a lower priority than a manually summarized one. An automatically summarized route does not carry path information because BGP summarizes only local routes that are imported using the import-route command. BGP4+ does not support automatic route summarization.
Manual route summarization	BGP routes are summarized manually.	The attributes carried in a manually summarized route are controllable. Whether to advertise the specific routes for summarization is controllable. During BGP route selection, a manually summarized route has a higher priority than an automatically summarized one. A manually summarized route can carry specific path information, which prevents routing loops. Both BGP and BGP4+ support manual route summarization.

An automatically summarized route comes from local routes, and the mechanism of automatic route summarization is much less complex than that of manual route summarization. Therefore, the next section describes only manual route summarization.

Atomic_Aggregate: a well-known discretionary BGP attribute, carried in Update messages, indicating that a route is a summarized one. BGP speakers cannot delete this attribute during route transmission.

Aggregator: an optional transitive attribute, carried in Update messages, indicating where routes are summarized. Aggregator consists of the AS number and router ID of the router that performs the route summarization.

AS_Sequence: a type of AS_Path, carried in Update messages, recording in reverse order all the numbers of the ASs that a route passes from the local device to the destination address.

AS_Set: a type of AS_Path, carried in Update messages, recording all the numbers of the ASs that a route passes from the local device to the destination address without an order. AS_Set can also indicate a summarized route and carry path information. Therefore, if a summarized route carries AS_Path, Atomic_Aggregate is optional. During route selection, a router considers that AS_Set carries only one AS number regardless of the actual number of ASs.

AS_Set affects BGP route selection. Whenever AS_Set changes, a router sends Update messages to its peers whose routes are not summarized by the router to notify the change. If the summarized route passes through a large number of ASs and the specific routes change frequently, the router needs to send Update messages frequently to its peers to notify them of the AS_Set changes. This process may lead to route flapping.

AS4_Path: a new attribute defined by BGP. It is similar to AS_Path in function, but AS4_Path can carry both 2-byte and 4-byte AS numbers. AS4_Path can be classified as AS4_Sequence or AS4_Set, which are respectively similar to AS_Sequence and AS_Set in function.

AS4_Aggregator: a new attribute defined by BGP. AS4_Aggregator carries 4-byte AS numbers, while Aggregator carries 2-byte AS numbers.