The Aspath Attribute
AS_PATH is a well-known mandatory attribute that uses a sequence of AS numbers to describe the inter-AS path, or route, to the destination specified by the NLRI. When a BGP speaker originates a route—when it advertises NLRI about a destination within its own AS—it adds its AS number to the AS_PATH. As subsequent BGP speakers advertise the route to external peers, they prepend their own AS numbers to the AS JPATH (see Figure 2-21). The result is that the AS_PATH describes all the autonomous systems it has passed through, beginning with the most recent AS and ending with the originating AS.
Figure 2-21 AS Numbers Are Prepended (Added to the Front of) the AS_PATH
Figure 2-21 AS Numbers Are Prepended (Added to the Front of) the AS_PATH
Note that a BGP router adds its AS number to the AS_PATH only when an Update is sent to a neighbor in another AS. That is, an AS number is prepended to the AS_PATH only when the route is being advertised between EBGP peers. If the route is being advertised between IBGP peers—peers within the same autonomous system—no AS number is added.
Usually, having multiple instances of the same AS number on the list would make no sense and would defeat the purpose of the AS_PATH attribute. In one case, however, adding multiple instances of a particular AS number to the AS_PATH proves useful. Remember that outgoing route advertisements directly influence incoming traffic. Normally, the route from the NAP to AS 100 in Figure 2-21 passes through AS 300 because the AS JPATH of that route is shorter. But what if the link to AS 200 is AS 100's preferred path for incoming traffic? The links along the (500,200,100) path might all be DS3, for example, whereas the links along the (300,100) path are only DS1. Or perhaps AS 200 is the primary provider, and AS 300 is only the backup provider. Outgoing traffic is sent to AS 200, so it is desired that incoming traffic follow the same path.
AS 100 can influence its incoming traffic by changing the AS_PATH of its advertised route (see Figure 2-22). By adding multiple instances of its own AS number to the list sent to AS 300, AS 100 can make routers at the NAP think that the (500,200,100) path is the shorter path. The procedure of adding extra AS numbers to the AS_PATH is called AS path prepending.
Figure 2-22 AS 100 Has Begun the AS_PATH Advertised to AS 300 with Multiple Instances of Its Own AS Number
The other function of the AS_PATH attribute, as discussed earlier in the chapter, is loop avoidance. The mechanism is very simple: If a BGP router receives a route from an external peer whose AS_PATH includes its own AS number, the router knows that the route has looped. Such a route is dropped.
As the name implies, this well-known mandatory attribute describes the IP address of the next-hop router on the path to the advertised destination. The IP address described by the BGP NEXT_HOP attribute is not always the address of a neighboring router. The following rules apply:
• If the advertising router and receiving router are in different autonomous systems (external peers), the NEXT_HOP is the IP address of the advertising router's interface.
• If the advertising router and the receiving router are in the same AS (internal peers), and the NLRI of the update refers to a destination within the same AS, the NEXT_HOP is the IP address of the neighbor that advertised the route.
Continue reading here: The Atomicaggregate and Aggregator Attributes
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