8.8 Issue with Using Private AS  Numbers
8.8.3
Why and how to manipulate AS_path information
AS_path information is manipulated to affect interdomain routing behavior. Because BGP prefers a shorter path over a longer one, system operators are tempted to change the path information by including dummy AS path numbers that would increase the path length and influence the traffic trajectory one way or the other. Cisco's implementation enables a user to insert AS numbers at the beginning of an AS_path to make the path length longer. The following example shows how this feature can be used.
Lab Activity
  In this lab, you will learn how to filter BGP updates based on the AS_path attribute.

Because BGP prefers a shorter path over a longer one, system operators can add to the AS path length by prepending extra path entries and influence the preferred route used by other ASs. The following example shows how this feature can be used.

In Figure , AS50 is connected to two providers, AS200 and AS100. AS100 is directly connected to an Internet Network Access Poin t (NAP), while AS200 has to go through an extra hop via AS300 to reach the Internet. Figure shows the AS path of prefix 192.213.1.0/24 as it traverses the ASs in its way to the NAP. When the 192.213.1.0/24 prefix reaches the NAP via AS300, it would have an AS_path of 300 200 50. If the same prefix reaches the NAP via AS100, it would have an AS_path of 100 50, which is shorter. ASs upstream from the NAP would prefer the shorter AS_path length and would direct their traffic toward AS100 at all times for destination 192.213.1.0/24.

AS50's network administrator is not too happy about this behavior because they prefer for Internet traffic to come in via their higher bandwidth T3 link to AS200, rather than through the slower link to AS100. AS50's network administrator can resolve this by manipulating the AS_path information, inserting extra AS hops when sending routing updates to AS100. One common practice is for AS50 to repeat its AS number as many times as necessary to tip the balance and make the path via AS200 become shorter.

In Figure , AS50 will insert two AS numbers 50 50 at the beginning of the AS_path of prefix 192.213.1.0/24. When the prefix 192.213.1.0/24 reaches the NAP via AS100, it would have the AS_path 100 50 50 50, which is longer than the AS_path 300 200 50 via AS300. ASs upstream of the NAP would prefer the shortest path and would direct the traffic toward AS300 for destination 192.213.1.0/24.

The bogus number should always be a duplicate of the AS announcing the route or the neighbor the route is learned from (in case an AS is increasing the path length for incoming updates). Adding any other number is misleading and could potentially lead to routing loops. Note the insertion point in Figure .