300-420 Question 31
Single answerA network engineer is tasked with ensuring that only specific routes from a branch office are advertised to the core network. The engineer decides to apply route filtering on a Border Gateway Protocol (BGP) router using a prefix list. Which action ensures only the desired routes are advertised?
- A
Create a prefix list to permit the desired routes and apply it to the BGP neighbor in the outbound direction.
- B
Apply a route map that denies all routes to the BGP neighbor in the inbound direction.
- C
Configure a distribute list with the desired routes and apply it under the OSPF process.
- D
Use an access control list (ACL) to filter outbound traffic on the router interface.
Show answer and explanation
Correct answer: A
Explanation
In BGP, route filtering can be implemented using prefix lists to control which routes are advertised or received. To advertise only specific routes, a prefix list permitting the desired routes must be created and applied to the BGP neighbor in the outbound direction. This ensures that only the permitted routes are sent to the neighbor, while other routes are filtered out.
- A. Correct.
Correct: A prefix list is commonly used in BGP to permit or deny specific routes. Applying it in the outbound direction ensures that only the desired routes are advertised to the BGP neighbor.
- B. Incorrect.
Incorrect: Denying all routes in the inbound direction would block all incoming route advertisements, which is not the intended goal. This would disrupt the BGP session entirely.
- C. Incorrect.
Incorrect: While distribute lists can filter routes, they are typically used with IGPs like OSPF or EIGRP, not BGP. This option does not apply to the scenario.
- D. Incorrect.
Incorrect: Access control lists (ACLs) are used for traffic filtering, not route filtering. They cannot influence which routes are advertised to BGP neighbors.