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Suppose ASs X and Z are not directly connected but instead are connected by AS Y. Further suppose that X has a peering agreement with Y, and that Y has a peering agreement with Z. Finally, suppose that Z wants to transit all of Y’s traffic but does not want to transit X’s traffic. Does BGP allow Z to implement this policy?

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Yes, with the help of BGP protocol Z can implement this policy directly.


Consider the following data:

The network consists of 3 Autonomous systems AS X, AS Y and AS Z.

AS X has an agreement of peering with AS Y.

AS Y has an agreement of peering with AS Z.

BGP (Border Gateway Protocol) is an Inter-AS routing protocol. It obtains the subnet reachability information from neighboring AS and propagates the reachability information to all routers within an AS. The peers determine the routes to each other AS system.

• The BGP protocol allows AS Z to implement the policy by the way the BGP route advertisements are handled by each AS.

• AS Y should advertise AS X that, it has no path to Z even though there is a link YZ.

• As the system AS X is unaware that AS Y has path to AS Z, AS X never forward the traffic.

• Hence, AS Z can transit all of Y’s traffic.

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