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The AS_PATH attribute has been presented so far as consisting of an ordered sequence of AS numbers that describes the path to a particular destination. There are actually two types of AS_PATH:

• AS_SEQUENCE—This is the ordered list of AS numbers, as previously described.

• AS_SET—This is an unordered list of the AS numbers along a path to a destination.

These two types are distinguished in the AS_PATH attribute with a type code, as described in the section "BGP Message Formats."

NOTE There are, in fact, four types of AS_PATH. See the section "Confederations" for details on the other two types: AS_CONFED_SEQUENCE and AS_CONFED_SET.

Recall that one of the major benefits of the AS_PATH is loop prevention. If a BGP speaker sees its own AS number in a received route from an external peer, it knows that a loop has occurred and ignores the route. When aggregation is performed, however, as in Figure 2-28, some AS_PATH detail is lost. As a result, the potential for a loop increases.

Suppose, for example, AS 810 in Figure 2-28 has an alternate connection to another AS (see Figure 2-30). The aggregate from AS 3113 is advertised to AS 6571, and from there back to AS 810.

Because the AS numbers "behind" the aggregation point are not included in the AS_PATH, AS 810 does not detect the potential loop. Next, suppose a network within AS 810, such as 206.25.225.0/24, fails. The routers within that AS will match the aggregate route from AS 6571, and a loop occurs.

Figure The Loss of Path Detail When Aggregating Can Cause Inter-AS Routing Loops

Figure The Loss of Path Detail When Aggregating Can Cause Inter-AS Routing Loops

If you think about it, the loop-prevention function of the AS_PATH does not require that the AS numbers be included in any particular order. All that is necessary is that a receiving router be able to recognize whether its own AS number is a part of the AS_PATH. This is where ASJSET comes in.

When a BGP speaker creates an aggregate from NLRI learned from other autonomous systems, it can include all those AS numbers in the AS_PATH as an AS_SET. For example, Figure 2-31 shows the network of Figure 2-28 with an AS_SET added to the aggregate route.

The aggregating router still begins an AS_SEQUENCE, so receiving routers can trace the path back to the aggregator, but an AS_SET is included to prevent routing loops. In this example, you also can see why the AS_SET is an unordered list. Behind the aggregator in AS 3113 are branching paths to the autonomous systems in which the aggregated routes reside. There is no way for an ordered list to describe these separate paths.

When an AS_SET is included in an AS_PATH, the ATOMIC_AGGREGATE does not have to be included with the aggregate. The AS_SET serves to notify downstream routers that aggregation has occurred and includes more information than the ATOMIC_AGGREGATE.

Figure 2-31 Including an AS_SET in the AS_PATH of an Aggregate Route Restores the Loop Avoidance That Was Lost in the Aggregation

Figure 2-31 Including an AS_SET in the AS_PATH of an Aggregate Route Restores the Loop Avoidance That Was Lost in the Aggregation

Like most options in life, AS_SET involves a trade-off. You already understand that one of the advantages of route summarization is route stability. If a network that belongs to the aggregate fails, the failure is not advertised beyond the aggregation point. If an AS_SET is included with the aggregate's AS_PATH, this stability is reduced. If the link to AS 225 in Figure 2-31 fails, for example, the AS_SET changes; this change is advertised beyond the aggregation point.

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