The Atomicaggregate and Aggregator Attributes

A BGP-speaking router can transmit overlapping routes to another BGP speaker. Overlapping routes are nonidentical routes that point to the same destination. For example, the routes 206.25.192.0/19 and 206.25.128.0/17 are overlapping. The first route is included in the second route, although the second route also points to other more-specific routes besides 206.25.192.0/19.

When making a best-path decision, a router always chooses the more-specific path. When advertising routes, however, the BGP speaker has several options for dealing with overlapping routes:

• Advertise both the more-specific and the less-specific route

• Advertise only the more-specific route

• Advertise only the nonoverlapping part of the route

• Aggregate the two routes and advertise the aggregate

• Advertise the less-specific route only

• Advertise neither route

Earlier, this chapter emphasized that when summarization (route aggregation) is performed, some route information is lost and routing can become less precise. When aggregation is performed in a BGP-speaking router, the information that is lost is path detail. Figure 2-28 illustrates this loss of path detail.

Figure 2-28 Aggregating BGP Routes Results in the Loss of Path Information

AS 3113

206.25.128.0/19

206.25.192.0/19

AS 23"

206.25.160

AS 3113

206.25.128.0/19

206.25.192.0/19

AS 23"

206.25.160

206.25.224.0/19 (810)

AS 225

AS 810

206.25.224.0/19

206.25.192.0/19

AS 3113 is advertising an aggregate address representing addresses in several autonomous systems. Because that AS is originating the aggregate, it includes only its own number in the AS_PATH. The path information to some of the more-specific prefixes represented by the aggregate is lost.

ATOMIC_AGGREGATE is a well-known discretionary attribute that is used to alert downstream routers that a loss of path information has occurred. Any time a BGP speaker summarizes more-specific routes into a less-specific aggregate (the fifth option in the preceding list), and path information is lost, the BGP speaker must attach the ATOMIC_AGGREGATE attribute to the aggregate route. Any downstream BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute cannot make any NLRI information of that route more specific, and when advertising the route to other peers, the ATOMIC_AGGREGATE attribute must remain attached.

When the ATOMIC_AGGREGATE attribute is set, the BGP speaker has the option of also attaching the AGGREGATOR attribute. This optional transitive attribute provides information about where the aggregation was performed by including the AS number and the IP address of the router that originated the aggregate route (see Figure 2-29). Cisco's implementation of BGP inserts the BGP router ID as the IP address in the attribute.

Figure 2-29 The ATOMIC_AGGREGATE Attribute Indicates That a Loss of Path Information Has Occurred, and the AGGREGATOR Attribute Indicates Where the Aggregation Occurred

Figure 2-29 The ATOMIC_AGGREGATE Attribute Indicates That a Loss of Path Information Has Occurred, and the AGGREGATOR Attribute Indicates Where the Aggregation Occurred

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