In the figure, router A in AS 64520 advertises network 192.168.1.0. When that route traverses AS 65500, router C prepends its own AS number to it. When 192.168.1.0 reaches router B, it has two AS numbers attached to it. From the perspective of router B, the path to reach 192.168.1.0 is (65500, 64520).
A similar process applies for the paths to networks 192.168.2.0 and 192.168.3.0. The path from router A to 192.168.2.0 is (65500, 65000), which means traverse AS 65500 and then AS 65000. Router C will have to traverse path (65000) to reach 192.168.2.0, and path (64520) to reach 192.168.1.0.
Building Scalable Cisco Internetworks (BSCI) v3.0
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