The ATOMIC_AGGREGATE Attribute

Route aggregation causes a loss of information because the aggregate is coming from different sources that have different attributes. The ATOMIC_AGGREGATE attribute is a well-known discretionary attribute (type code 6) that gets set as an indication of information loss. Basically, if a system propagates an aggregate that causes loss of information, it is required to attach the ATOMIC_AGGREGATE attribute to the route.

The ATOMIC_AGGREGATE should not be set when the aggregate carries some extra information that gives an indication of where the aggregated information came. An example is an aggregate with the AS-SET parameter, as discussed earlier. An aggregate that carries the set of ASs that form the aggregate is not required to attach the ATOMIC_AGGREGATE attribute.

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Troubleshooting:  
Example: Ch. 10, pp. 343-346. Aggregate Only, Suppressing the More Specific

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The AGGREGATOR Attribute

The AGGREGATOR attribute is an optional transitive attribute (type code 7). It specifies the autonomous system and the router that has generated an aggregate. A BGP speaker that performs route aggregation might add the AGGREGATOR attribute, which contains the speaker's AS number and IP address. In Cisco's implementation, the IP address is actually the Router ID (RID), which is the highest IP address on the router or the loopback address if it exists. The loopback interface is the virtual interface discussed earlier in this chapter. Figure 5-22 illustrates the AGGREGATOR attribute. AS300 is receiving routes 192.213.1.0/24 and 192.213.2.0/24 from AS100 and AS200, respectively. When RTA generates aggregate 192.213.0.0/16, it has the option of including the AGGREGATOR attribute, which consists of the AS number 300 and the RID 193.0.34.1 of the router (RTA) that originated the aggregate.

Figure 5-22  See AGGREGATOR implementation example.

The ORIGIN Attribute

The ORIGIN attribute is a well-known mandatory attribute (type code 1). It indicates the origin of the routing update (NLRI, which indicates prefix and mask) with respect to the autonomous system that originated it. BGP considers three types of origins:

•  IGP—The Network Layer Reachability Information (NLRI) is internal to the originating AS.

•  EGP—The Network Layer Reachability Information is learned via the Exterior Gateway Protocol (EGP).

•  INCOMPLETE—The Network Layer Reachability Information is learned by some other means.

BGP considers the ORIGIN attribute in its decision-making process to establish a preference ranking among multiple routes. Specifically, BGP prefers the path with the lowest origin type, where IGP is lower than EGP, and EGP is lower than INCOMPLETE. For more details on how the ORIGIN attribute is calculated, refer to the section, "ORIGIN of Routes," earlier in this chapter.

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Notes:  
The originator ID and cluster list attributes are discussed in Chapter 8.

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BGP Decision Process Summary

BGP bases its decision process on the attribute values. When faced with multiple routes to the same destination, BGP chooses the best route for routing traffic toward the destination. The following process summarizes how BGP chooses the best route.

1.  If the next hop is inaccessible, the route is ignored (this is why it is important to have an IGP route to the next hop).

2.  Prefer the path with the largest weight (weight is a Cisco proprietary parameter).

3.  If the weights are the same, prefer the route with the largest local preference.

4.  If the routes have the same local preference prefer the route that was locally originated (originated by this router).

5.  If the local preference is the same, prefer the route with the shortest AS_path.

6.  If the AS_path length is the same, prefer the route with the lowest origin type (where IGP is lower than EGP, and EGP is lower than INCOMPLETE).

7.  If the origin type is the same, prefer the route with the lowest MED.

8.  If the routes have the same MED, prefer the route in the following manner: External (EBGP) is better than Confederation External which is better than Internal (IBGP). Confederations will be explained in Chapter 8.

9.  If all the preceding scenarios are identical, prefer the route that can be reached via the closest IGP neighbor—that is, take the shortest internal path inside the AS to reach the destination (follow the shortest path to the BGP NEXT_HOP).

10.  If the internal path is the same, the BGP router ID will be a tie breaker. Prefer the route coming from the BGP router with the lowest router ID. The router ID is usually the highest IP address on the router or the loopback (virtual) address. The router ID could be implementation specific.