In Figure 4.42, suppose that there is another stub network V that is a customer of ISP A. Suppose that B and C have a peering relationship, and A is a customer of both B and C. Suppose that A would like to have the traffic destined to W to come from B only, and the traffic destined to V from either B or C. How should A advertise its routes to B and C? What AS routes does C receive?
Consider the data from the Figure 4.42:
Number of provider networks=3 (A, B and C).
Number of customer networks=4 (W, V, X and Y ).
Autonomous system AS A collect the information only from AS B.
Autonomous system AS V A collect the information only from AS B and AS C.
To decide the information as AS W, need to transfer from AS A to AS B at the AS-PATH. The peering relationship contains AS B and AS C. Then AS C route the packets to AS B to AS A. Thus, AS C follows the AS-PATH CBAW.
So, the route to be advertised by A for B to reach A is AS-PATH AW. The path used by C to reach W is AS-PATH CBAW.