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Example: Null AS Path

AS 1 and AS 3 are connected to AS 2, which you administrate. AS 3 advertises its routes to your AS, but you do not want to advertise AS 3 routes to AS 1 and thereby begin routing traffic from AS 1 to AS 3 through your AS. To prevent this situation from occurring, you can configure an export policy for AS 1 (1.2.2.6) that allows routes for your AS to be advertised to AS 1 but does not allow routes for AS 3 or routes for any other connected AS to be advertised to AS 1:



[edit policy-options] 
null-as "()";
policy-statement only-my-routes {


term just-my-as {


from {
protocol bgp;
as-path null-as;
}


then accept;
}




term nothing-else {
then reject;
}


}
protocol {


bgp {


neighbor 10.2.2.6 {
export only-my-routes;
}


}


}

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