Refer to the exhibit. An engineer has been asked to redesign the traffic flow toward AS 111 coming from AS 500. Traffic destined to AS 111 network 91.7.0.0/16 should come in via AS 100, while…

By prepending the AS path on the 91.7.0.0/16 advertisement sent to AS 200 (the unwanted inbound path), AS 111 makes that route appear less attractive to AS 500 due to a longer AS path length - since AS path length is evaluated before MED in the BGP decision process. AS 500 then selects the path through AS 100 for 91.7.0.0/16 while all other prefixes continue to use the existing path through AS 200 because they carry no additional prepends.

Not advertising 91.7.0.0/16 to the bilateral peer would make that network completely unreachable via that peer, which is not the goal - the goal is to influence which path inbound traffic uses, not to withdraw reachability.

Local preference is an internal BGP attribute used within a single AS to influence outbound path selection; it is not propagated to external BGP peers and cannot influence how AS 500 sends traffic toward AS 111.

A higher MED advertised to AS 100 would signal to AS 100 that the path into AS 111 via that link is less preferred, which would discourage traffic from entering via AS 100 - the opposite of the desired behavior.